Why Janet Van Dyne isn't insane after such a long stay in the Quantum Realm
Time and isolation would be your worst enemy. Madness would likely consume you after several years, as desperation would go to hopelessness and then to suicide. Think about people stranded on remote islands or in desolate locations like the Sahara Desert or Antarctica. The passage of time is inexorable. Remember those Thai children stuck in that flooded cave recently? A lot of psychologists were surprised that they didn't end up dying, because they said that most human beings in such a situation generally give up hope after about ten days.
Then you have the Quantum Realm, where time and space likely have different meanings.
Perhaps time goes much, much slower in the Quantum Realm than in real time. A day there might be year on Earth. In Quantum Realm time, Janet might have been trapped there for only 4-5 weeks! That explanation would answer one big, lingering questions at movie's end concerning her intact mind. In that same period of time, she might have been unaware that she was aging decades, even though she must have been aware that Hank and Hope had aged decades. Her reactions seem to bear this out. Again, the movement of time in the Quantum Realm would be a valid explanation.
Thoughts? Hypotheses? Feedback?