&gt;It baffles me that an independent, older woman with an already established career is willing to date a younger guy that can’t offer her much right now.

oh god, that's darling, she's 23 and you're 21

OK. I need more details on why this guy is so horrible, because this:

&gt;He’s not very good looking, he wasn’t a nice person in high school, and he has a history of just looking to have sex with girls. 

sounds like 95% of teenage boys. None of that is terribly shocking, there must be more. Did he murder small animals? Did he rape someone? Did he lie, is he a racist homophobe?

&gt;He and my other friends cited the fact that she used to date him as a red-flag about this girl. 

OK, I could possibly see that if she knew he was a murderer or a child molester or a huge racist and dated him anyway. But again, nothing you've said about him really convinces me that he's the devil, so...

&gt;Simply put, I feel like our relationship is less special due to the fact she dated this jerk. Am I crazy?

yes, that is crazy. They dated for all of five months, ages ago. Maybe he put on a good face at first and convinced her he was amazing. You need to get over it.

EDIT:

also, why do you think this?

&gt;It sounds like she was basically used for sex and that’s hard as hell to hear. 

why couldn't *she* have been using *him* for sex? Like, did she outright *say* that she thought he used her for sex?

&gt;However, she explicitly told me she doesn’t regret the relationship, which took me aback.

again, why should she? nothing you've said about this guy is particularly notable as being so much worse than the average dude brah one finds in literally any small town.