You need to have confidence in order to make a change in your behavior, especially a big change like in compulsive overeating, emotional eating, or mindless eating. Having confidence in your ability to change your eating makes you optimistic about going on a diet - a big plus!
Let's say you're low on confidence at this point. Here are 5 ways you can go about building self confidence:
Review past successes in other areas of your life
Consider the personal strengths that gave you those successes
Reframe the idea of past diet "failures" into the idea of past diet "tries"
Think about past methods as needing revision
Revise the methods so they will work better next time
Don't think you are a hopeless case just because you've tried and failed. Trying is an essential step on the way to succeeding.