You can beg to be demoted but, you tell your employer why and you won't have a job in 6 months. Sure, they can't fire you because you're mentally ill, instead they'll just find some other reason to rid of you, perhaps they've already trained your replacement, not hacking it with the promotion? Yea, that's a good enough reason.

My advice? Deal with it. Life is stressful.

You really didn't think your promotion would be stressful? Some part of you wants to lose your job maybe?