How do I tell my mom that I don't want to be in band anymore?

My mom is a fanatic about marching band, but I'm not. I've played clarinet for three years but I've been faking it. I don't understand key signatures and can't play high notes. I don't want to start over or learn a new band instrument, it's just not my thing. What I really want to do is learn piano. I played for a little bit when I was little but quit after a while. The biggest problem is that I got invited to come on a music tour of Europe and play with others all around Europe, and I would kind of like to go. It wouldn't be fair for me to go when someone that can actually play an instrument can't. Plus, my family is a little money-deprived right now. Not poor or broke, just money-deprived. My brother has medical problems and we've had to go across the country to get him evaluated and such. I tried telling my mom that I want to quit but she keeps saying I'll regret it. How can I convince her to let me quit?