I'm afraid that if I submit a piece to this literature magazine at school, that I'll get rejected.  If I get rejected, I think it'll crush me.  How do I convince myself that if I don't get accepted, that doesn't mean I'm rubbish at writing?
submit your work to many, many places. the first ones will be just practice. then if you get a chance you should probably make your own magazine, because then you get to decide what is 'good'