Weight gain

Over a couple of years I managed to lose six stone and felt fantastic. I now seemed to have, almost overnight, put two stone back on. I feel such a failure and am determined to lose it, but it is soo hard! I have hypertension so it is very important to tackle this. But my self loathing is surely out of proportion. I write down everything I eat and try not to over eat, few cakes, no chocolate or crisps, regular exercise. What am I doing g wrong? I hate myself for letting this happen