hi @Cleo2. If these people at work were using hate speech to you and to customers then I tend to think you dodged a bit of a bullet there by not working there anymore. Although I completely understand how disheartening it must be. They don't sound like the kind of people an explanation is going to mean anything to. Their reaction proves that I think. It all sounds incredibly immature, cruel, not to mention unprofessional to me.
I would be very concerned about where the people at the volunteer position heard those things about you as well... that's terrible! Again, it doesn't sound like an environment I would want anything to do with.
You have a history of mental health issues, like a lot of us, that is never an excuse to be treated like how you have been, which seems to be on the extreme end of the 'nasty' scale. You definitely deserve to be treated better. "better" doesn't'' even seem to cover it here. Its hard to imagine who a person could be treated worse in a workplace. Even treated like a human being would be an improvement.
I;m sure the mental heal support group will be far, far more supportive... it would be difficult not to be. But I think you'll find it very helpful and find it reassuring to be around people who aren't so... horrible! Let us know how you get on with it. And sorry again for how you were treated in those two positions.