It’s 9 March 2017. I am sitting in an ambulance, holding a plastic cup which contains tea from a machine. I’ve just been told my son is dead. I’m in a kind of paralysis. I feel the cold, smooth vinyl of the trolley I’m sitting on beneath me and look vacantly at the equipment and signs around me. Mind Your Head. Sharps Bin. No Smoking. Clinical Waste.

I am alone, apart from a paramedic who is with me. At this particular moment my world has shrunk to the inside of the ambulance. An alienating sense of shock and horror has taken over and reduced me to a stiff and silent state. After a while I am helped out of the vehicle and taken to a nearby Victorian building. I am led into a wood-panelled room where I sit on a sofa.

Here I wait. It is dark outside. My husband is more than 200 miles away and it is already 7pm. A friend is on the way and I am told we have to wait for the police as well. This is a sudden, unexpected death of a young student who has not been seen for a few days. They will need to investigate.

Nearly four years on and I am standing at the top of a hill in Devon which overlooks the River Dart estuary. It is low tide and the river is a silver dribble in the middle, with mud flats either side. I can see the sea in the distance and over to my left is the dark silhouette of Dartmoor.

Ranks of rectangular mounds are dotted over the hillside. This is Sharpham Meadow, where Felix rests. I feel lucky that we were able to bury him here. It is a place which has meaning for us. It connects, via the river, to our home upstream on Dartmoor and downstream to the sea, where we spent so much time as a family.

This cold winter’s day, my visit is short. The wind is howling a gale as it usually does up here and I don’t feel like lingering. There is a sole, very early, primrose in flower on his grave, looking rather bedraggled. I stand for a few minutes, looking down the estuary and thinking about times past: swimming down at Castle Cove in Dartmouth at the mouth of the river; walking along the cliffs on the other side of the estuary mouth, where Felix and his brother Lucian played inside the Daymark, an old stone tower built to guide sailors into the harbour.

On warmer days, when the tide is high, I often go down to the river after visiting Felix and swim in the estuary from where I can look up and see “his” hill. Somehow, by immersing myself in the water, I return to a sort of primal, elemental essence, and I feel a profound connection to him. It’s as though I become part of what he is now, a body in the ground. I think of that line from Genesis: “Dust thou art and unto dust shalt thou return.”

Felix, was born in 1997, my first child. I never knew there was room for so much more love in my life. He was an adorable blob with thick, dark hair. Motherhood was a bit of a shock, as I think it is for most people. He struggled to feed, and, aged just two weeks, went back into hospital after a diagnosis of “gross failure to thrive”. He had to be force-fed through a tube, but he came through it and soon gained weight.

And so started a beautifully normal childhood. The black hair he was born with fell out, to be replaced with thick blond locks which fell into a straight, pudding bowl style. He was a hilarious toddler. His favourite thing, aged about three, was to sit and “play” the piano. He would bash the keys and sing, pausing every so often to turn the page of the music, as he had seen me do. Then he would turn around and look at his audience, in anticipation of the inevitable claps. The usual stages followed. Nursery, primary school, secondary school. Normal family life.

Then, one night in 2010, Lucian came running into our bedroom. Felix, who was then 13, had fallen out of bed. We rushed into his room to find him unconscious on the floor, jerking. We called an ambulance and by the time it arrived the convulsing had stopped. It was clear he had suffered a seizure. We were told it might be a one-off, but sadly it wasn’t and he started having seizures regularly, both while asleep and awake.

During Felix’s teenage years we had lots of visits to the paediatrician and then the neurologist, experimenting with medication, different dosages, and so on. His seizures were never properly controlled. It was tough for him and affected his confidence, but throughout it all he never complained or asked, “Why me?” He found it more difficult to make friends as he grew older and he became quite isolated, spending a lot of time in his room. I was very worried about him.

Despite his problems, Felix got a place at the University of Leicester. He had a false start studying law, and then changed to film and history of art. While spending time at home before starting afresh at Leicester with the new course, he joined a community theatre group on Dartmoor and began to find his feet. They welcomed and nurtured him, and he gained confidence, enough confidence to put himself forward for auditions when he started again at university.

I was so proud of him. To put himself through auditions and go on stage, when he could have a seizure at any time, was quite an achievement. He also started writing scripts and loved being part of the university theatre scene, making friends and belonging. It was brilliant. For my beautiful son, who’d suffered so much, life was at last coming good.

We couldn’t make it to his first university theatrical performance, which was in Antony and Cleopatra, set in a nightclub with a drag queen playing Cleopatra. His godfather, Sean, had that particular honour. However, I was excited about going to see his next production, which was the musical, The Producers.

I’d arranged to meet Felix a few hours before the show, but when I got there he wasn’t there. I waited around for a bit, but he didn’t show. I tried ringing him. No reply. I found the theatre where I knew they were rehearsing and was told they’d been trying to track him down for a few days.

I started to feel an icy fear in my stomach, but tried to stay calm. There would be an explanation, surely. We phoned his hall of residence and were told someone would go and check his room and call us back. I waited, trying to suppress a growing feeling of panic, and heard nothing. So I decided to drive there myself. The traffic was frustratingly slow. Eventually, I arrived and the first thing I saw was an ambulance. Feeling sick with fear I leapt out of the car and ran over to his building where there were several people, including a paramedic outside. They wouldn’t let me in and they wouldn’t tell me what had happened. I tried to push past them, but the paramedic stepped forward and said: “It’s not very nice in there, I wouldn’t.”

Then another paramedic emerged from the building and put his arms round me and said: “I’m very sorry. He’s passed over.”

I was taken to the ambulance where I was given a cup of tea and from that point started the new phase of my life. The phase without my precious, beautiful son in it.

A postmortem concluded Felix’s death was due to SUDEP – Sudden Unexpected Death in Epilepsy. The charity SUDEP Action defines it as when, “A person with epilepsy dies suddenly and prematurely and no reason for death is found.” The cause is not yet known. Researchers are investigating a range of possibilities, such as the effect of seizures on breathing and the heart. The deaths usually occur at night and are unwitnessed. Felix’s body was found slumped down the side of his bed. Three people with epilepsy die every day in the UK and it is one of the most common neurological conditions in the world. And yet few people have heard of SUDEP.

The risk of SUDEP was never mentioned by any of the doctors or nurses who treated Felix. And yet it turned out that he was in a high-risk group, being a young male adult who had seizures in his sleep. The main risk factors for SUDEP are as follows: having generalised tonic-clonic seizures; having nocturnal seizures, being a young adult, developing epilepsy before the age of 16, and being male. Felix was all of these.

Who knows if his death could have been prevented? Certainly, if his seizures had been better controlled, he would have been less at risk. Throughout his life his care had been patchy at best and perhaps if it had been more consistent, he might have had fewer seizures. Thousands of young lives are cut short every year across the world because of SUDEP. Many of those who die are, like Felix, students, away from home for the first time. SUDEP Action believes these deaths could be reduced with better education. After all, if you don’t know about the risks, you can’t do anything to reduce them.

Dealing with Felix’s death has been the biggest challenge of my life. Soon after he died, I found myself writing letters to him. I couldn’t accept that he had died. It was a way of keeping him alive, of perpetuating the fantasy that this hadn’t actually happened. I told him what he meant to me, I told him how much I missed him, how angry I was at the injustice of his death. I told him about the banalities of my life and I told him about how the world was taking the news of his death. I told him in detail about his funeral. I shared my daily experiences with him, my walks on Dartmoor and the coast, and my swims in the River Dart that I found increasingly beneficial in dealing with the grief. I asked him lots of questions. Where was he now? Could he still feel things? What was death like? Was he in pain? Did he know when he was dying?

My memory was fogged by grief, which I found deeply upsetting. My memories of Felix were all I had left, and yet I felt him fading away. It was as though I were in a boat, looking down at him in the water. Gradually he was sinking, then drowning, slowly being consumed by the water and falling out of sight.

Now, nearly four years on, I am less tortured by the physical absence of Felix. I have learnt that he is always a part of me, deep inside me, just like when he started as a tiny seed in my body. What matters is his presence in me. When I want a kind of physical, visceral experience of him, I return to the river, where, somehow, in the water, I become a non-person, an absence, like him.

For more information on SUDEP, go to sudep.org.

Sophie Pierce’s memoir about losing Felix, The Green Hill: Letters to a Son, is currently crowdfunding with Unbound. To find out more, visit unbound.com