Sorry I can't reply in a song, I'm pretty crap at artistic things. Anyway, it seems like life has been pretty hard for you. Despite how hard you're working to get better from physical injury, friend problems etc. you're getting knocked back down. Unfortunately I'm not the sporty type either so I can't exactly offer you specific advice on that. All I can say is that I've been a similar situation where even though I'm doing everything I can, I'm never quite good enough or never quite there yet. ITS HELL. The thing that has got me through some of the lowest times is that:
Life is unfair, you can't do anything about it, go ahead and have a bit of a cry or angry outburst over it, but the all you can do is get up, wipe the dust off and keep going.
I know the hurt seems never ending but it will get better!
I'm not sure if talking to someone will help you get some of this frustration or other emotions off your chest. Is there a parent, friend, school counsellor, relative, family friend, coworker or anyone you feel comfortable talking with? I'm assuming you're not very comfortable with talking openly about stuff because you've stated the only way you can express your situation and feelings is through a song. Might be worth a try though. Lifeline (13 11 14) and Kids Helpline (1800 55 1800) are also great listeners if you just want to get things off your chest.
It seems as though you have a lot of emotions held up inside of you. Maybe doing something to release it might help. Music and songs are a great way of expressing your feelings as you've shown really well. I'm not sure how physically okay you are at the moment but could something like boxing or something intense like that help. Maybe even screaming into a pillow or something, there are a lot of different things like that you can try. You can't lose much by giving it a go.
Remember that you're braver than you believe, stronger than you seem and smarter than you think.