I really want to bring up polya to my fiance again, but last time we ended up separating for a month. But I feel so...not me and idk what to do.
Don't stay in a relationship that makes you feel "not you." A healthy relationship lets you fully express the whole of who you are, in a positive and growthful way. If you feel like this because you can't live into your polyamory, maybe this relationship isn't for you. And it sounds like your fiance is very very mono. That's fine - it doesn't mean that your fiance is bad, or wrong, just that you two aren't compatible. It's a sad and frustrating thing to learn, but it's good information to have before you decide to get married.