Do you realise how many people on this site will not be able to sympathise with your plight?! Don't worry, I know exactly how you feel. At 13/14 I was the same height I am now (5ft11") and about 7 stone- basically I looked like a beanpole. Then accelerated puberty hit and bam! I got boobs, hips, a bum and now have to struggle to keep the weight off!
I would relish your frame and high metabolism now, as you may not always be so lucky...
It's really good that you like sport- keep this up, as quitting exercise will not help you fatten up.
Could you tell me what your everyday diet consists of? That will give me a better idea of how to advise you.