Idk about you but for me this turned out to be just a weird looking fear of commitment. It felt like fomo, but once I realized what it was, it .. well, I didn't stop thinking about other girls right away, but it lost its power to make me actually lose interest in the girl I was with.
I'm still with the girl I was with when I realized all this, so, imo that's pretty meaningful.
(Fwiw I never stopped "thinking" about other girls but now it's very unreal. I've been with my wife for 15 years and the idea of going back through all that intro relationship bullshit sounds like torture -- plus, odds are, whoever she is won't be better than my wife, and, of course, it would super hurt my wife's feelings, and probably mine too, and I really like her feelings not to be hurt. Also we have kids, which raises the bar for how shitty the relationship with my wife would have to be in order to spilt up.)