What caused me to give it up?
I had three strikes against me: I am not rich, I am shy and untrusting, and I have chronic hand pain. I think I might have made it through a program if anyone of those handicaps had gone away.
I don't know how graduate school is supposed to work. I don't know how anyone learns anything in those programs--but obviously they do. I went to classes that felt like a waste of time, did my homework, and never managed to have a real conversation with anyone in the department, student or professor. I might as well not have shown up--and it would have been nice if someone had told me that. I think they should warn undergraduates about...whatever mysterious system graduate school uses. Certainly no one has ever explained it to me.
I still don't know what I did wrong, but as far as I can tell it has nothing to do with my ability to do physics. So I buy textbooks and read them for fun; I slowly study QM and group theory and topology. I've found hundreds of errata in Von Neumann, solved nearly every problem in the first six chapters of Bransden and Joachain, every problem in Chapters 1-3 of Spivak's Calculus on Manifolds, studied general relativity (and forgotten it again probably eight times.) I love physics. But I'm not a social person, and that apparently is the kiss of death. I'm not smart enough to do it all on my own--I can't replicate an entire graduate program in my spare time. I'll likely die of old age before I ever master enough of the prerequisites to dig into quantum field theory. Anything I find interesting, has prerequisites for the prerequisites for the prerequisites for the subject that I have to learn. And there is the eternal race against forgetting, running from diff eq to linear algebra to vector calculus to QM to complex calculus and back to diff eq's because I'm forgetting again, round and round. I can't keep the knowledge because I never get enough of it in my head at once to DO anything with it.
I'll never be a physicist, which was my dream. Instead I seem to have become the teacher I never had. I help a lot of students, and I love my work as a tutor, and they tell me I'm amazing, and it makes me happy to help them. Maybe some day I will write textbooks. I've thought about writing "Spivak for the Merely Brilliant" for example.
Why did I have to quit? I'll probably never know why. Dreams die hard, if at all. But you find a new dream, and you go on.
Apologies if this was TMI, but you *did* ask.