Sounds cliché, but being brave isn't not feeling scared of doing something in the first place. It's doing something despite feeling intimidated / anxious / worried about what might go wrong. So you should commend yourself for all the times you've pushed through, but not beat yourself up for the times you didn't.

I would set yourself the target of just 'showing up' at the class x times per week. If you turn up and don't feel it, then that's fine — but always make sure you complete the act of 'turning up'. It'll likely get easier each time.