Hey there,
I can relate to what you're going through, because I've been in your situation. Mine was slightly different though, the guy lead me on and treated me like trash once he started seeing another chick. I have severe depression, and I feel that I can offer a unique perspective on what you're facing.
First of all, I get how you feel. Honestly. You feel useless, clingy and like your worth depends on what this one girl thinks of you. It's dehumanazing. I'm not going to bitch about your post history, because honestly, everyone has been obsessive at one point in their lives. I've done it, and most of them have done it too. I'm also not going to pressure you to "move on", because it's easier said than done and quite frankly, I don't think any words to that effect will make a difference to you. All I can say, is that you'll be happy. Trust me. The first thing you need to do is go no contact and realise that this girl just doesn't deserve you. She doesn't deserve your constant waiting on her, and no amount of care you give can make up for what she lacks towards you. You need to go no contact. If seeing a counsellor helps, do just that. See your GP and ask about support groups in your local area, or speak to someone you trust. Do whatever you need to be happy, but don't jeapordise her relationship in the process. It's not fair on her or her boyfriend.
Would you consider seeing someone else?