Very good advice particularly from Al.
My 2p worth:
1) It's really hard with your first love. You do a lot of things for the first time (not just sex). The rush of feel-good chemicals in your brain when you're in love can be extremely powerful, and the feeling and memories really gets under your skin (in a great way when it's going well, and in a "I'm grasping a burning hot coal" painful kind of way when it's not).
2) Why has Havard not prescribed a lemsip and a w**k? Is he feeling OK?
3) I do think you're better off without her. That's really cliched and sounds straight out of the agony-aunt pages, but I believe it to be true. I refer to Al's excellent advice here. Sure, she has depression and probably needs support to help her with that. Her treating you like dirt may be a symptom of that, but it won't help her or you.
4) You probably hate her right now for posting up that stuff about seeing another guy. I think she's done you a favour by saying that. It's clarified that she's either using emotional blackmail to win you back (and that story's unlikely to have a happy ending, because I can't see that behaviour changing for a very long time) or b) she is very quick to move on, which suggests she was either seeing someone else already, or she is desperate to have a boyfriend. Needing a boyfriend/girlfriend is different from finding someone you want to spend your life with. One starts with "I need a guy/girl, I'll go out and find one", and the other is "Wow, you're fantastic, can we get to know each other better?"
Try not to hate her. I don't think she was with you for the intention of using you. Love is tricky. She's dealing with the breakup in the way she knows. So must you.