Hello, I wanted to get your opinion on an issue I just had with my fiancé. So I have an appointment today that my fiancé agreed to drive me to yesterday and this morning. However, he changed his mind earlier because he wanted to buy weed and didn’t want to drive twice. So he drove to buy weed and came back, refusing to drive me to my appointment later.
I got so angry because he chose to drive to the store to buy weed instead of drive me to my appointment in a few hours. He could still drive again to drop me off, but no. He decided he was going to buy weed and not drive me anymore.
I got so angry I started screaming when he got back, because he didn’t understand what was wrong with what he did. In our argument he admitted he was selfish. He thinks I’m overreacting and maybe I am. I’m not mad because he didn’t drive me. I’m mad because he chose to drive himself to buy weed instead of driving me later.

I feel like I’m crazy for losing my sh*t. I just need someone to give me their opinion, I don’t care who it is.

Nah, I'm like this as well with my mother and her boyfriend. Though the situation is much more different.

I think you did overreact yelling the second he came back. You probably should go to him about it and apologize for behaving wildly. It could have been a much calmer type of argument to start out with. Immediately raising your voice isn't very effective from my experience. The fact he bought weed but ignored your request was insanely selfish. He had no real reason to NOT drive you other than being a dick. But there's not much you can do about it now.

TDLR; Yes he was a dick, but you shouldn't have yelled. Approaching him and discussing it would have been a much better approach. But we're human and everyone makes mistakes. Apologize to him but still address his actions to work out a compromise for future reference. Communication is key in relationsips.