How to let go of a ‘what if’ relationship?

I need help and advice, how do you let go of a ‘what if’ kind of relationship- the kind of feeling that you hold on to what could have been with someone?
I have a partner who I love deeply and I want a future with him…BUT there is a guy from my past that I didn’t date but I’ve always thought about and wondered ‘what if?’.

We’ve always been friendly and I wanted more but it’s never been the right time, he’s married now (living apart) but he was being flirty with me recently and I’m unsure about how to let that ‘what if’ feeling go, which is very unfair to my partner, I don’t want to hurt him and I would never cheat- but there is a part of me that still wonders what it would be like with the guy from my past…any advice would be great.

How to let go of a ‘what if’ relationship?
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