Ever have to get your nerve up & quit?

[smilingcat]

Here's what I take away from your post:
You're reacting because you're p*ssed at your boss.
You love the girls and want to keep volunteering with that.
Maybe you have boundary issues & it's hard to say "No" as you keep "subbing" for classes. Try saying "Sorry, I'm not available" instead of subbing.
Keep up your primary spinning classes only (no subbing).
Make time for yourself and don't feel guilty.
Good luck. You're worth it.

Agree with Velocivixen. Great advice. Managers are people too and they take the easy way out. If you are too accommodating, they WILL take advantage of you. And they have.

Be strong and take care of yourself first. Let your injury heal. Take care of yourself emotionally then the rest of the things in your life.