Overweight thirteen year old, please help?

Okay, hi. I'm thirteen and I'm 158 cm tall, and 55 kgs. I had a medical last week because when I was eleven I had an eating disorder, and still need check ups -.- Anyway, I've gained 5 kgs since Summer (I'm Australian) and hate it. I'm in the 75th percentile for weight, which means 75 percent of girls my age are lighter than me. Yoiks! So I stopped eating sugary food, and started doing that thing where you wear a rubber band on your wrist and snap it when you want something. My Vitamin C, Iron, and Vitamin D all come out of pills as I'm vegetarian. All the sports I do are indoor (Netball, Swimming and Taekwondo). Taekwondo is my life, though. I started about a month ago and practically fell in love with it from the second I started training. I have two tape "stripes", one for punching and one for basic poomsai. Anyway, I can't go for the grading because I started too late. That means I stay a white belt for another term -.- It's going to kill me watching the other white belts become yellow (they all have at least three years on me anyway but still) and having to stay back. There's one other white belt that's staying back, but he's 29 and so fat he literally can't fit into a dobak. Anyway, I just want to be perfect. Help?