First off, I don't think you should call it a DIET. That just kills it because you'll feel like it's something you have to 'get off of". Make this a lifestyle change, not just to fit into that dress but to be healthy and feel good about yourself for the rest of your life.
-Just try to get into the mindset! set yourself a goal and stick it on the wall, write it on your hand. REMIND yourself why you're doing this and remember...we all mess up, it's what makes us human but we can also fix our mistakes too. Just know that you have it in you to do it. Get determined, push yourself and don't make failure an option. You can do this!!!
just remember that you're not the only one.