I think everyone kinda glossing over the fact that OP here was 18 when she met her fiance who was 28 at the time. Some of the stuff fiance has done is manipulative I feel. " It turns out we had a spark and I was able to talk her into ending it with her girlfriend and start seeing me " - I'm not sure that's exactly a green flag

" I wasn't even interested in anyone, honestly looking back I just wanted to hurt Amy. " - At this point OP's fiance is what, 35? 36? That's a fully grown man failing to communicate that he doesn't like this arrangement.

I'm not sure if psycho is the right word for it, but asshole definitely is