Dieting is like climbing a rope. If climbing a rope is hard for you ( ...and dieting is hard for everyone ), there are three things you'll need to get to the top: building your skill, having success, and feeling self-confident.
These three things are intertwined. You can't get one without the others. Think of it this way - you know you can't climb the whole rope. so what do you do. You try to climb a little part of it. That takes less confidence and skill than climbing the whole way up at once.
If you are successful in climbing the first small part of the rope, then your confidence increases. As your confidence increases, you feel more like going ahead to develop the skill you need to climb higher.