How can I be relationship material (vs. being FWB/side chick material)?

It seems like I’m the one that man will happily sleep with but now want to commit to. This has always confused me. I don’t sleep around (I can count the number of men I’ve slept with on one hand) and I refuse to be in casual relationships (I’m very upfront about this).

The last guy I was involved with begged for me to give him a chance (after rejecting him multiple times). When I gave him a chance, he told me that he only saw me as a friends with benefits (stating that we weren’t compatible because I’m not confident). He would tell me about the women that he was interested in dating and he would make me feel inferior (“with women I want to date, I take them out to go do things, I don’t tell them to come over”). When I ended things after four weeks because I refuse to be a FWB and after finding out that we had a girlfriend the entire time, I blocked him and he refused to leave me alone (kept calling me “No Caller ID”, kept looking me up on LinkedIn). One day, he approached me at a friend’s house and asked me why I cut him off. I told him that I don’t know if he’s playing dumb or what, but I found out that he was already involved with someone. He then responded, “I just wanted to hear you say it”.

Although I don’t want to be with him, I keep comparing myself to him girlfriend. Wondering why I didn’t measure up and what she has that I don’t. Why was she relationship material and I wasn’t? Why she was good enough to commit to, meet his friends and family, take on dates and vacations, and be his +1, meanwhile, I got treated like crap.