It’s been over a decade, so the exact article where I first read about Mort’s angst over Superman and his rumored nervous breakdown has been lost to time. That said, a Google search of “Mort Weisinger” and “nervous breakdown” gave me some interesting articles on the topic like this interview with his son Hendre Weisinger.
This pretty savage take on Mort, his character, and how it lead to the “miracle conceits” of his Superman books:
https://www.hoodedutilitarian.com/2009/03/letter-to-the-believer-about-mort-weisinger/
And, I’ve never read this book, but the description mentions Weisinger’s nervous breakdown:
I wish I could find the article I originally read about the topic because it laid it out beautifully, but, again, it’s been over a decade. If I find it, I’ll post it here, but I will say that interview with his son had some pretty illuminating quotes:
Weisinger: Do you remember the Superman cover where Clark Kent is saying to Superman, “Can you get yourself a new identity? I’m through with you.”
Alex: Right.
Weisinger: Okay. That, as a psychologist, was exactly a projection of how my father was feeling at the time. He was getting sick, he wanted to do other things.