When I had my first serious encounter with psychosis, my entire personality changed. I went from being extremely gregarious and uninhibited to shy and socially anxious. I also went from being an average student to someone who couldn't stand to get anything lower than an A.

When my depression got to the point where I couldn't keep my A average, I became very laid back and lazy. I went from being a perfectionist to someone who could only produce sloppy work.

Since getting on a medication that works, I am slowly regaining my confidence in both social and academic contexts. I am relearning how to relate to people, and how to work hard and put effort into things.

So yes, mental illness and its treatment can cause lasting change in someone's personality.