Was this the woman who slept with the other guy during your trip together?

Obviously you're not REALLY attracted to her. Every time you see you, you absolutely resist getting sexual with her. (Except for when you force yourself to beg for a kiss at the last moment).

I think you never actually cared until that other guy showed how quick & easy it was to have sex with her. Then you got a little jealous, felt like you missed out on an opportunity, and decided you deserve to have sex with her also since you had invested so much more time into building some rapport.

Now that it's been a year and you've been spent it stewing over this missed opportunity for a travel fling, your mind has transformed her into some fantasy and you've got a form of oneitis.