I came from exactly your situation 6 weeks ago, the only difference being that once her ex found out about us, he wanted her back and she went back. We dated for about three weeks, and we started out as best friends before dating. Anyways, once he knew she was with me he started to call and tell her things like " your the only girl I ever loved", I think you get the picture. She told me he didnt mean a thing and not to worry. this behavior went on for about a week. So, one day I called her up to make some plans and she told me she was confused. In so many words I told her that the fact she needed to make a choice was reason enough for me to leave, and thats what I did.
So my advice to you is this: If your already having probs of this nature after three weeks, move on. Cause if you don't, she prolly will. And you'll really feel bad. I say to hell with the whole lets be friends deal, or trying to be there for her. She's not there for you and so she says that she knows its hurting you, then why is she doing it. She will see you as weak, and desperate by hanging on to her, while she really wants to be with that other jerk off. Do you really think you could live with that, knowing her thoughts were with him, while being with you. Leave now, the writings on the wall. She is obviously not interested. Save your dignity.
You sound like a nice guy, and I also was the dreaded "nice guy". But overall she will see you as desperate.