The problem: Otherwise known an 'all or nothing syndrome', this is what happens when you tell yourself you've blown it with just one slice of chocolate cake - so you finish the whole thing.
The solution: A lapse isn't a collapse! Just get straight back on track with healthy eating. If allowing yourself a small portion of your favourite indulgence is too hard, you may need to keep foods that are a weakness completely out of the way for a while. You need to eat over 12 Mars bars (3500 calories) to put on 1lb of fat. So one bad day isn't going to spell disaster.