This is perfect! It's honest, transparent, but not cruel or intentionally hurtful. Saying "the chemistry isn't right" doesn't make it sound like you think he's an awful person at all, just that it isn't a match, no blame or anything.

If you're comfortable with it, I'd say exactly what Golden_Eye is recommending!

OK, that aside... it's already been said, but you're absolutely not under any obligation to go out with him again. You were caught off-guard and trying to get out of a situation gracefully. You're dealing with your own mental health issues. You're human, and even if you said yes enthusiastically - you're allowed to change your mind or reconsider!

I think going out with him again would just make this harder, for both of you.

I get the anxiety (oh boy, do I ever!) but does it help to think of this like... building a muscle, almost? It's hard and awkward and a lot of work, but standing up for yourself by communicating in this situation helps you get practice doing that, which will make it easier to do in the future.

Good luck!