There are some practical considerations I’d have (you’ll probably have to take the MCAT again since the scores tend to expire after a certain number of years, how would this affect finances for school, etc.) but it all comes down to what you can live with and only you can answer that.
If you knew without a doubt you wouldn’t be a doctor if you join PC, that going means you’ll have to find another career path after your service, would you still join? Could you be at peace with that? If yes, go to PC. If you know you’d never be happy unless you’re a doctor, go to medical school. That’s what I’d ask myself, and force honesty from myself on what I want, not what I should do. But no one can answer those questions truthfully except you.
From what you wrote, I’ll say you don’t sound mentally ready to go to medical school right now. Maybe it’s just a particularly bad time, but your tone suggests you’re not feeling the onslaught of challenges medical school brings right now. There’s nothing wrong with that, and in fact everything right in recognizing it.
Not to throw another wrench into play or be a Debbie Downer, but many friends have come back from Peace Corps a bit … defeated might be a strong word. But service is certainly no walk in the park. It can be a constant uphill battle to get projects implemented, handle culture shock, feel like you’re making any bit of difference, etc. If you’re exhausted now, is there a way to request a one-year deferral from the medical school and Peace Corps, stay where you are, recover emotionally and see where you stand in a year?