I am writing an MA thesis right now after a 20 year hiatus in my graduate studies and the difference in my mood about it is just amazing. One big difference is I am more confident and wise about the whole thing. But the other big thing is how other people have tended to react to my work. I keep being told in professional contexts how “accomplished” I am with my studies and I was accepted to present at a prestigious academic conference over the summer. When I was in my 20s I never got that treatment. I find the whole thing really interesting and, yes, I had a successful career in between so maybe that is what people are reacting to. But my studying just gets a lot more respect than it did when I started (I’m finishing literally the same degree I started in the 90s so it’s the exact same thing I was working on then).
All my friends who did PhDs got out of the field eventually and are SO much more mentally healthy than they were, even the ones who were successfully publishing and teaching at universities.