Is it a bad idea to quit right after a promotion for a lesser paying job due to mental health concerns?

I recently got promoted in a field that I had genuinely enjoyed when I first started. I was hoping for this promotion early on but the way I’m getting it now is worrying and stressing me out a lot. I’m essentially swapping positions with a colleague who’s been underperforming, and they’re meant to train me on their role.

As you can imagine, that’s a bit awkward as this person is essentially getting demoted and I’m taking their spot, which is causing them to not be the most proactive in training me. My boss wanted the training completed by end of April and the closest training we have is booked for the last week of April. I stressed this to my boss who kept saying he’ll talk to my coworker but of course nothing has happened yet. I know 100% I will fail at this new role because of my lack of training. I’m now considering quitting and looking for work elsewhere and I have come to terms that I’m willing to accept the more mundane/repetitive jobs (e.g: data entry) which is about 20k-30k less than what I make now. My mental health has completely deteriorated in the last few weeks from getting this promotion knowing that I’m essentially set up to fail. I can afford to live without a salary for about 2 months. Is this a bad idea? I don’t know how long I’d take a lesser paying job, but I don’t know how to build myself back up resume-wise if I decided to this.