Forgive yourself, and try again in a way that does not set you up to fail. Using the diet example: don't go all-in to a strict diet. Start making small, achievable changes. Drink more water daily, and maintain that for a few weeks. When it is second nature, add another simple goal to master.
Remember, keep each goal achievable, and keep on working on it even if you give in to temptation sometimes. Forgive yourself, and keep going. :)