It's super subtle and insidious how it starts to take hold of you. You don't even realize you're losing yourself as it first happens.
I had gotten straight a's all through college, then started eating meth as a way to stay up and study in my senior year. My grades dropped from a 4.0 to a 3.7 in 1 term, and all my teachers began to look at me as a slacker and irresponsible. I also began to show apathy towards everything.
I didn't even realize it was the meth until it was too late. I wasn't addicted and I didn't do it regularly. It'll sneak up on you.
That said, Meth isn't as evil as the mainstream narrative would suggest for everybody. I know many people who own houses, maintain jobs, and live crime-free. She's a tricky bitch.