I try to eat healthily, but recently I've been bingeing on junk food and put on a stone. I go to the gym four times a week and do an hour of cardio plus weights. What's going wrong?

While you appear to be physically committed to exercise as part of a weight management strategy, I fear your brain is not. In my experience, this is quite common with individuals who struggle with their weight. It creates a behaviour that negates one's efforts, as you start to doubt the outcome of your actions. Be careful - this can create a cycle of low-calorie, quick-fix diets in an attempt to see results fast. Quick results are often short-lived and serve only to strengthen what I call a template of failure. Regardless of your efforts, you may perceive you will still be in the same situation and this makes it harder for you to move away from bad eating habits. Seeing lasting results involves changing to nurturing a template of success.

This does not happen overnight, but to start I suggest you pick three small things that can be building blocks each week. These need to be achievable but slightly challenging. For example, operate my carb curfew, where you eat no bread, pasta, rice or potatoes after 5pm, or ensure you drink two litres of water little and often through the day. Simple strategies, but consistency builds confidence and success. I'd also suggest you look beyond your four weekly sessions and pay more attention to your activity levels outside the gym. There are 168 hours in a week; four hours of exercise can be a very small proportion of potential physical movement.

· Joanna Hall is a fitness expert (joannahall.com). Send your exercise questions to Weekend, 119 Farringdon Road, London EC1R 3ER (weekend@theguardian.com).