One way of using psychology to lose weight is to build in opportunities for plenty of success.
Having success means reaching your goals as much as possible. The more you reach your goals the more successful you feel. The goals have to be realistic though. They can’t be meaningless or be way below your level of challenge. They can be small and readily achievable, just as long as there is some challenge to them. They can be sub-goals of larger goals.
Success can be measured objectively as in reaching some tangible goal. But it can also be measured subjectively by achieving something more personal, something on the inside. You could say that subjective success is comprised of thoughts and feelings that are also tangible, but they are only tangible internally to you. A goal of not being so blind about yourself and your foibles is as good a success once you reach it as not taking that extra helping of food.