Why can't I play the trombone?

I've been trying to play the trombone-I play it in the school band-for about a year now. At first I didn't care, but a few months ago I really started to care. But no matter how hard I try, I can't play most high notes and every time I play a sound, it's really scratchy. There's nothing wrong with my instrument-I checked-and I refuse to switch instruments. I've worked too hard and plus, I love the trombone. What can I do?