What's a good diet plan? I need some help, like now?

I'm Only 13... And I weigh 180. I can loose 20 pounds then the next week have it, then loose it again! I barely ever eat that many sweets. I hate cake and marsh mellows and super sugary stuff i can't stand it. Way to sweet! I like broccily, brussel sprouts, carrots, blueberries and all the stuff "normal" teenagers hate. I have no clue why this is happening! I could use some help. Like now. I mean its not that i Eat wrong (like i explained) and i also am good in sprts i never fail a fitnesss test like in sit ups the least ive ever done was in 3rd and i only did 46 and that was because i was sick! :( This is like hard! Is there any thing that will help!