Mono-poly relationship 30M, 30F

My partner (30M) is poly and I (30F)am not. My partner recently realized they were poly. We are monogamous. I was proposed to while we were monogamist. Now they are poly. We have been together for 3.5 years and they are the love of my life, and they say I am theirs. Well, They recently told me they had caught feeling for a mutual friend and I am STRUGGLING. We love each other, but I have become so insecure and feel self conscious in myself. We’ve talked about it, and they’ve cut contact with our friend, but I feel that my partner doesn’t want to live monogamy, and is settling for me. I have tried spicing things up in the bedroom, being more adventurous. I’m trying to keep my partner happy, but I feel like they are not happy anymore because I don’t want to share them with other people. I love my partner, and they say they love me, but I don’t know how to go forward because I feel like they aren’t happy with just me anymore. I would love some advice from others in similar situation and how to go forward with our relationship?