Or it could be both.

Personally I struggled, had tons of stress, no time for relationships, and all that. But it was also an intense time of personal development. I learned a lot about myself and a lot about how to work hard and get stuff done.

I used to tell people that if you don’t regret starting your PhD when you’re halfway through, then you’re not doing it right. I’ve since moderated that stance - a lot - but there’s still something to be said for so-called “character building experiences” even though they suck at the time.