Hi genung,
I say go for it. It seems to me that you have slowly been moving in that direction for years. So what if you finish residency when you're 56 or 60; you may well still have 20 years of practice left. And even if you don't, so what, you'll be doing what you're really interested in doing during the next 8 years.
And yes, school/residency will be hard but what else is new: you've already made it through PhD psych school/residency and NP program. I also believe that you being a psychologist and a nurse practitioner will make you VERY competitive to MD programs
(btw, don't forget about DO programs!).
The only caution I would have is if you just want full scope of practice and believe that you can't have that as a PhD/NP but only as an MD. Things are changing and in many states nurse practitioners have full independent/full formulary scope and of course, psychologists are also moving in that direction. So, if all you want is full scope and are ok with moving then I would suggest consideration of an alternative to the MD/DO route.
BUT, if your self-concept has evolved to seeing yourself most professionally fulfilled as a psychiatrist physician, then again I suggest that you don't let anyone dissuade you from that MD/DO path.