Hi sadibird, welcome to the board.
From what you have said, I think you should keep your exploration of sexual/intimate feelings to a more limited level until you get your feelings sorted out.
I met my ex husband and commited to him very young, age 19. We got engaged after a couple years, but after the NRE wore off with him, I did find myself getting lots of crushes on other people (men or women) and when that happened, I usually did lose desire for my partner. However, he was extremely jealous, and threatened by my roving eye.
Later in life, I found that my interest in others would also increase my desire for my husband. We had worked thru some of his jealousy issues by then.
I have 3 kids about your age. My oldest who is now 25 had a poly relationship for 5 years, but she found it too complicated to pursue after a while. She broke up with that person and is now mono with a new partner, for now at least. Poly is complicated, and "kids" in their 20s are still doing lots of self-exploration. If you don't know yourself very well, it's hard to be respectful and thoughtful about others. Not impossible, but it takes a lot of work to get to enjoy the considerable benefits.
Maybe you and your fiance can explore polyamorous feelings thru fantasy and role playing for while as you work things out? Be as open and honest as you can, revealing your feelings in a respectful way, and see where it leads down the road.
Lots of women are hornier than their mates. If you need to masturbate a lot when your man is not in the mood, let him know this, and make sure he supports it without guilting you.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 34), miss pixi's Dom for 3 years