I keep coming off my diet.?

I became overweight at the age of 11 and started losing weight when I was 13. Then when I was 14 I put it back on. Then when I was 14 nearly 15 I lost weight again really fast, and then gained it back within a few months. By the age of just over 15 and a half I lost weight again and became very skinny. When I was 17 I once again gained it all back plus more and became the heaviest I have ever been weighing 111 kilos. I started dieting at the start of this year and have got down to 89 kilos and still have atleast another 30 kilos to lose. The problem is I keep going off my diet. I will go on my diet for a week then I will come off of it for 3-4 days. So I have stayed at the same weight for about 2 months now, infact I even gained 2 kilos at one point, but lost it again. When I go off my diet I eat so much. I just need help.