I fell in love with my best friend while engaded...

First of all english is not my first language so i'm sorry if there are any mistakes.

I (23F) am engaded to my wonderfull fiancée (27F), we have been together for more then three years. I love her dearly and I truly want to spend the rest of my life with her. She's polyamorous however I dont know if I am. This was something we both knew this before we started dating. For me the idea to love more than one person is beautiful and I would love to share my love and life with many people, but unfortunately I'm a jealous person which makes me think I'm monogamous. My fiancée said that she's ok with that and that she's ready to be in a mono relationship with me. I've always been sceptical about it, because I'm afraid she will be unhappy with me in the future, but that's her choice not mine. I've known my best friend (25F) for almost four years and she's an incredible person. Lately i catch myself thinking about her more in a romantic way other than platonic, but I'm 99.9999999% sure she has never seen me this way. I told my fiancée about this and at first she thought it was cute and teased me about it, but after some talking she actually admited to having mixed feelings about it. She thinks its unfair because when she got a crush on a girl I had mixed feelings (her crush was lowkey rude to me which is why I had mixed feelings) and she felt guilty. I agree IT IS unfair and I really wish i could go back to the way it used to be. I don't want to ruin my friendship and I don't know what to do with those feelings, but the only two people I would feel comfortable talking about this is my best friend (and I obviously cannot tell her) and my fiancée (but it makes her a little sad so I can't). I'm hoping for some advice or that at least typing this will ease my mind a little. I feel like I can lose them both because of my stupidity, while all I want to do is cuddle with both of them... :(((