I am with Harrington, she either still has feelings for you or feels uncomfortable (because of the idea that she hurt you for example). I used to be very lonely as well, though I had tons of friends. But I didn't have a partner and that made me feel pretty bad (or at least, I thought it was). Having a girlfriend is wonderful, but only when both of you are completely comfortable with each other. Trust me on that one. And the same goes with friends. You can't be good friends when one side is not comfortable. What I guess is you want to be friends for you sake, and true friendship is care for each other. It is extremely hard to let go of someone who makes you feel wanted and gives you a purpose to live, but you might want to consider her feelings as well. Yes, her boyfriend might be a cheater, a liar, and whatever, but ask yourself the question whose decision it is that she wants to be with him.
Perhaps you can start with small steps. It is quite the jump if you are going from lonely to girlfriend in one step, skipping the "someone I know", "a friend", "good friend" and "really good friend" stages. And in general, friends can be a huge support, and are more likely to stay than a partner, since you have more and have a less intense relation. Of course, a girlfriend can add things to your life that nothing else can.
A thought I like is: "How can you truly love someone when you can't love yourself?" Can you be the partner that someone deserves when you are afraid of losing her? Pretty tough questions, and black and white since you probably both end up helping each other, but it might be something worth thinking about
Sorry if this sounds like a lecture or something
, I can get quite caught up in rambling. Just try to relax a bit, it will come.