I wouldn't say I regret going to med school, although I probably wouldn't have done it if I had known then what I know now about what medical training would be like. But I didn't know then what I know now, so it doesn't really matter what I would have done if I had been "fully informed."
I will say this: practicing as a physician is very different than training to be a physician. As an attending, you have a lot more ability to control your work and personal life. Don't like your current job? Vote with your feet. Go do a fellowship. (That's what I'm doing.) Join an academic program and become an educator. Work abroad. Cut your hours. There are lots of options, particularly if you are flexible geographically and/or willing to accept lower income.
In contrast, training really does suck a lot of the time, not the least of which because you are pretty much stuck where you are, working long hours whether you want to or not, until you finish. People should know that fact going in. If they don't really, really want to see this thing through to the end, so much so that they are willing to go to hell and back for all those years in training, then they should reconsider their career plans. You can't dabble in becoming a physician. It's all or nothing. So if you don't want to go all-in, you should go for nothing.
And yes, PAs do have a much easier time of it during training, and are probably happier on average than residents are. Comparing the happiness of PAs to attendings is tougher. I don't think my PAs are inherently happier at work than I am. That being said, their job is not equivalent to mine, even though there is significant overlap. I would second the suggestion above that if you are not sure whether you want to be a doc vs a PA that you spend some time shadowing both. Residency does come to an end for physicians, so I wouldn't solely base your decision on what resident life is like. But it's still a factor worth considering when you're pondering whether going to med school is "worth it."