Well, what snapped me out was when I ran an experiment that proved how strong the placebo effect could be, which caused me to reflect on my beliefs and realize that literally all the Werewolf wizard powers I thought I had could be explained by the placebo effect. Naturally, I concluded that I couldnt trust anything my senses told me and spent a few days trying to figure out how to deal with the possibility of being a brain in a jar.

And of course, right after I'd rebuilt my entire conception of reality from first principles, that's when I found out that some of the memories I had of things I was most proud of and defined myself by were provably false. So, as you would expect from me considering my calm and careful reaction to the placebo effect, I then decided that all my memories couldn't be trusted.

So, can't trust my senses, cant trust my memories. That's pretty much all the things I can use to define myself. So, based on the lack of valid evidence I concluded that I do not exist.

And that's how I stopped being a flat-earther wizard werewolf. Thankfully eventually I came around to agreeing with Descartes on the whole "I think, therefore I am" thing. After I climbed out of the psychological hole I dug over the next six months, I recovered with only a severely crippling fear of advertisements.