~ CONTRADICTION: STORY OF MY LIFE ~

I am 15 years old and tired of not being able to go into a store and find things to fit me. I made the decision this year to finally lose the weight and be a happier person. Everytime I start to get a handle on diet and excersize something inside wants to ruin that and I will go back to my old waysof eating unhealthy and getting no excersize. I dont understand the contradiction in my head. It is a constant battle with myself. To be healthy, I HAVE to lose about 120 lbs. Thats not what scares me, the thing that scares me is that I wont be able to do it and I will like this forever. I need some support.