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.