Reach out to your PSL partner tell them exactly what you said here! That it's not them, you just feel really shitty right now and you don't want to get their hopes up.

I mean, if it were me, I'd definitely appreciate the honesty! Yeah it sucks to hear "Sorry, I just flaked bc my mental health is shit," but it's understandable, and anyone who doesn't understand that is a bit of a dick. I know personally I'd much rather prefer to hear "Sorry I dropped the ball, and I can't promise that I won't again in the future," than total radio silence, which leaves me sitting there wondering what I did wrong.