It's Saturday morning and we're lying in bed listening to the radio. It is lovely, the kind of thing that rarely happens, and I'm already nostalgic for these times that won't exist when we are living apart. The boys are watching television downstairs, and their sister is still sleeping.

"When shall we tell them?" asks R.

"I think we should speak to them separately," I say. Our middle child is the most likely to take the news of our separation the hardest.

I think my daughter – with her previous outbursts of rancour and confusion at my hesitation about leaving R, despite him drinking again – will probably be relieved that I have finally come to a decision. And the youngest barely talks in proper sentences, so it's difficult to see if we should tell him at all.

"OK. Well, I've got to go to work in a couple of hours and I don't want us to break the news, make everyone cry and then bugger off," R says.

We call down the stairs for our older son to come up with his brother. They come into the room and one plays on the floor with the laundry basket, while the other climbs into the middle of the bed, nuzzles into his father's armpit. "Strange Hill High's on the TV. Can you hurry up, please?" he says. I begin, and it is painful from the start.

"You know that Daddy had a break last summer, to try to sort things out? Well, things haven't really gone to plan," I say.

I wait, hoping that R will step in, and luckily, because I hate myself for breaking such horrid news, he does.

"When you come back from your holiday, I'm going to be living in a different place, just around the corner."

"You mean you aren't going to live here any more?" our son says, slowly working out what we have known for days. A lump in my throat begins to form, and the solid ledge – an imaginary line that stretches across my diaphragm and suppresses all of the emotions that I find too visceral to deal with – gives way. I don't want to let my son see how sad I am because I'm afraid that if I allow myself to cry properly, I won't be able to stop.

R, whom I have only seen cry once in our relationship (he sobbed emphatically as he gave his speech at our wedding reception), continues: "Yes, darling. Mum and I are going to be living in different houses soon. But I will still see you lots and you probably won't even notice that things have changed."

Our son doesn't buy this at all.

"But it won't be the same. I don't want you to go. I don't want you to go, Dad."

He is sobbing loudly and our youngest stops throwing underwear across the floor and looks up.

"Is there anything I can do to make you stay? Is there anything at all I can do? Anything. Please."

This is a million times worse than I imagined. I don't know why I pictured a calmer, less emotionally charged scene. He is saying things that children are scripted to say in films. It seems unreal. And both R and I feel helpless, hopeless. No, there is nothing you can do, I say, digging my fingernails into my arm to pinch the pain away. I'm listening to his pleas, remembering the powerlessness of being a child. We lie there, four of us by the time we've hoisted our youngest on to the bed. We wait for something to happen and I wonder if there is anything we can do to stay together like this for the whole day.

All I can do is hold our son and stroke his head, as I did when he was a baby. R takes his free arm, the one that is not jiggling the other boy up and down like a farmer on his horse, and he takes my hand and squeezes it.

Our daughter walks in and she's cross with all of us. "What's the matter? Why are you all crying?" she asks.

"Come here. Sit on the bed," says R softly.

"God, no. You're just going to tell me that you and Mum are getting a divorce. I'm going for a shower," she says.

"I know you're angry at both of us, but …" I begin. But she's already left the room and I hear the slam of the bathroom door.

"Just leave her. We can talk to her later," says R.

I want to call a friend and tell them I'm having a breakdown so I can stay in bed all day and cry. Or book a babysitter so R and I can spend a few hours together walking around, as a couple, in case we never go back to being one again. I'm scared to let R go. But I really must.