Tl;dr below

We are 19 and met almost a year ago. After a few initial fun dates it became clear that we were great for each other, but didn't immediately become a couple because she wanted to heal from a past relationship (from half a year ago). I respected her wish and waited until she was ready. During this time, however, I caught her texting with her ex, which seemed counter to her aim of forgetting him and made me a little uneasy. After we officially became a couple I started finding more about that relationship and how abusive and toxic her ex was...

He had started asking her for sex as soon as they became a couple, yet would only agree to go on a regular date very rarely. He treated her awfully for no reason, almost physically hurting her quite badly, and even confessed to going on dates with other girls (obviously he was doing more with them and just lying...in that sense, my girl got really lucky she avoided STDs). Nevertheless, she still allowed him to be her first... She took this treatment for two whole years until he broke up with her.

Any guy can see what happened here - he only wanted to use her for sex, and won big time. It is clear that she was naive and made a mistake. What bothers me is that she doesn't seem to understand the gravity of this mistake, as she still interacts with him on social media and until fairly recently refused to block his messages to her. From the beginning this guy wanted to get in bed with her and succeeded. She was used as a sex doll, treated with meager respect and dignity, and risked STDs. She ought to be infuriated with him, yet she isn't. The reason she tolerated this treatment for so long is because, according to her, she was seduced by the fantasy that her first relationship would be her only one...

Her relationship with me is very different, because I really like her. However at times I wonder if the fact that I'm not her first is acting in my detriment. She was strongly infatuated with that fantasy, but whatever I do I can't be her first. I just can't understand why she can't condemn him and say a mental "fuck you" to him. A person who used you for sex and from whom you risked STDs is not someone whose texts you answer and whose social media posts you like!

We have talked about her previous relationship a lot, though I don't think I can bring this up to her. I can't force her to resent him after all, she has to do that herself. I've done my best to give her insight on how the male brain works and what animals some men can be...but she just doesn't seem to understand.

In essence, despite being as good to her as I can I feel like I am missing an attribute that's very important to her - being her first boyfriend. Furthermore, I hate how this fantasy is blinding her to the point where she is still giving this douchebag a moral pass. I don't have any more energy to discuss her first relationship with her anymore...what should I do, if anything?

Tl;dr: girlfriend still goes easy on highly abusive ex, who among other things almost cost her her physical health, because she ascribes importance to first relationships