Okay first off, I have no idea what the decision for you to make. Most likely will never. Here's why:

Your decision comes from your heart. It can be tough when you have two things that you love and you can only pick one. You've put time and money into band yes? You've been practicing everyday correct? Then keep it up. Practice what you love to do.

This doesn't mean I'm saying to do band. If you want to try equestrian again, go for it! Trying new things is great and very rewarding. Getting frustrated and stressed simply means that you want to do it and that you want to get it right. Think of a river when this happens. A river that carries your stress and frustrations away. If you like to do it, do it!

The fact that your mother supports it (even if she thinks of wanting you to do something else) means that she respects your choice and what you might do. She will be fine either way and will help you. Now of course, without the pressure of parents, it can get hard to decide on what you want. All I am saying is that if you truly want to do the equestrian club then you will learn to make time for it and still get the good grades.

And oof, I need to stop typing and typing lol. Sorry this is so long ;)

This choice should not be determined by, "what if the barn sucks" "what if I dissapoint everyone" "what if I dont make it on the team".

I think it should be determined by, "What will make me (meaning you) happy?"

When I read your text, it seemed like you really wanted to ride, but were worried about the "what ifs" of band.

My suggestion is to sit down and think about both of them, which one makes you happier to think about, to imagine yourself doing, and to do!

Animals are my happy place, I havent ever felt frustrated when riding a horse.

Maybe its your riding instructor? Theres a lot of teaching styles, some will work awesome, and some really wont.

Thats really awesome that your mom supports you ^^

She will want you to be happy, so explain to her that you've considered your options, what you've decided, and that you think that the choice you've made makes you happy (or something along those lines :)