Curious question, which is a bit more scientific I suppose. What actually happens to your brain when you take stimulants long-term, neurochemically I suppose? What changes happen in the brain and how does this affect one's physical health and personality?

I have been taking stimulants almost daily for eight years now, and I am currently at around 60mg of Addy per day (give or take.) In addition to what goes on in the mind if I were to continue taking stimulants at this rate, and at this dosage, what would likely happen to me? I am 26 now, started when I was 18. Part of the reason I ask is that I notice I feel very off in my brain recently. I have a lot of weird, bizarre thoughts, am tire a lot, and seem like I am constantly running on fumes. I have also completely lost my ability ti be productive. Speed used to help me with my hobbies (art.) Now, whenever I get home from work, I engage in some meaningless, repititious activity for hours on end.