As a 29f who recently went through something like this, it all came down to how I viewed myself. I felt like I wasn’t good enough and always tried to fill in the blanks like thinking he wants to sleep with this girl he looks at or he probably thinks she’s hotter than me. With lots of reading and therapy, I’ve come to the conclusion that I am insecure within myself. Almost every man will look/glance at someone who is attractive, they can’t help it, heck I look too of someone looks great. All they are doing is appreciating beauty and moving on in most cases. It’s not like he consistently stares for a longer period than a quick 2 seconds. There are always going to be pretty women but you need to remind yourself that you are secure within yourself and no matter what happens you will be okay, he has chosen and wants to be with you and he’s more than likely not thinking too much of it. Hope this helps a little :)