Why is it so hard for me to get into shape?

I workout every day at the gym and occasionally (okay once a month) run a 6 mile course at a lake by my house. I'm 128 pounds, 16 years old and 5''5. I've played soccer for 10 years but have quit this year. This girl that I'm uming is around 140 pounds and about 5'3'' who eats out everyday at school and says she doesnt excercise at all (shes a bit chubby) can run faster times than me in track&cross country and ive been coming in the VERY last in all my meets. WTF is wrong with me im giving it my all and the problem is i USED to be faster.