Help me choose /adv/

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Hi guys, need some advice and to get this off my chest. I've been in a relationship for almost 3 years now, two of them living together, and we got engaged a few months ago. We have always being very serious about us and we have our whole future planned already. The thing is that we were open (we can have sex with anyone we want to as long as there are no real feelings involved) untill we got engaged, basicly bc he got too jealous of this guy. Since the day I met him (let's call him D), about a year ago, we had this explosive chemistry and were really atracted to eachother, but I do love my boyfriend , so I started to deny any feelings for D other than sexual atracction to make it "right" to have sex with him. Well, god-fucking-tier sex to be precise. D and I got to know eachother and became close friends, I also find out he's actually really similar to me and we are both fucked up in the same way.

After a few months my bf started to hate D, we had a huge argument and almost broke up, so I agreed to be monogamous. I talked to D and finally told him the truth I did not wanted to accept, that I loved him, but my boyfriend too, and my relationship is too important to let it end. Now I can't stop thinking about D, knowing that he loves me back and holding really deep feelings for him, to the point that I think I love him more than my fiancé, but I'm too afraid to break up this relationship, wich is nice and quietly happy.

I don't know what to do, follow my heat and leave with someone I deeply love and take the risk to get damaged or stay where I am and go on with my life?