My fiancée (51F) and I (48M) (I proposed two weeks ago) have a non-monogamous relationship. It's not an open relationship, our rule has been that we play together or not at all.

COVID and assorted health issues and time pressures have prevented us from "playing" at all for the last several years. All told, there has been substantially less outside play than I had hoped for when we first got together (I ended the relationship I had with the woman before my fiancée specifically because she was never going to agree to non-monogamy). Otherwise, the relationship has been great. We have some issues regarding vast differences in our parenting styles, which had prevented us from moving in together until about six months ago. Everything was moving along nicely, and I proposed. She said yes.

A week later, I learned that an attractive female friend is unhappy in her marriage and was looking for some validation. My fiancée has shut down any talk of a threesome with this woman (the phrase "skinny bitch" may have been used). I get that she doesn't want to be compared to this woman, and is afraid that I will leave her for someone more attractive. I'd like to believe I wouldn't.

The issue is, I can't help feeling like I'm missing out. I know, on its surface, it's a very stupid, selfish, self-destructive position to take, and the timing could not have been worse. I want to protect my future with the woman I love. How can I stop wanting what I've always wanted? My two desires are mutually exclusive - how do I let the destructive one go?

Please be as gentle with me as you can. I'm feeling very frightened and fragile right now.