This may, or may not, come across as patronising but, as a 30 year old man to a 20 year old man, I've been there.
My first really serious girlfriend was around the same age, we dated for four years before she suddenly started to withdraw on me, lose interest and then broke up with me (for a second time). I was devastated, I thought it would last forever, few months later she was bumping uglies with somebody else and all of a sudden the whole spiel about "best friend" yadda yadda, all changed, she left. It hurt big style, I was convinced I'd found my soul mate, that I was ready for commitment and so on, I think with our first loves it is ALWAYS going to be how we feel and if I am being honest, it'll be how you feel for MOST partners.
After all, why are you with them if you're not wanting to theoretically be with them forever? I don't date people as a partner if I see no potential in them.
The thing here is, you're the one on the receiving end of the rejection. As if often the case in PTSD. It wasn't your choice nor your desire to break up, it was hers, you're left feeling rejected and used and that is going to really hurt and it's going to mess with your emotions because whilst she may have disengaged emotionally, hence breaking it off, you haven't, you didn't get that memo and you didn't get rejected with any care for your feelings.
You're left picking up the broken pieces that YOU did not break, and it absolutely f*cking sucks.
My advice? Cut her out of your life. You see many stories here about PTSD partners and I honestly believe there's so much varying degrees but yours is on an extreme end of a scale where he behaviour is just completely out of line. It's not "just PTSD", theres way more going on there, she hasn't cared for you one iota and to be honest has treated you appallingly.
You deserve better.
Let her go and make her own mistakes, she's going to have to learn they're hers and no hero is coming to pick up the tab.
You need to disengage and work on you and healing yourself, you're going to feel guilty for a while yet and compelled to help her, but you MUST resist this urge. I feel that your relationship isn't the kind we see here where it'd be healthy, if only they sought treatment, it could work, it feels like it's beyond repair and I am telling you that, not to be cruel, but because I don't want to see you chase somebody who doesn't treat you with any respect.
Don't allow her lies to make you feel the guilt of a person who intentionally hurt her, you know you didn't, don't let her make you doubt yourself.