She's taken and so am I. What should I do?

Okay, here's the story:

Ive known her for years (as in maybe, 4-5 years?) And when we first met we both liked each other a lot. She had a friend ask me out for her. I didn't really even know what involved "going out" but I knew I wanted to. But my dad thought he was going to get a permanent job in Virginia (like 4 states away) and we were going to move there. So that's what I told her and that was that. We both were really sad, but accepted it and sort of moved on. And wouldn'tcha know it my dad didn't get the job and we stayed. Well actually we traveled around for like almost 2 years for my dad's nuclear plant outages he was doing. Anyway we stayed in touch and I knew she was still special. We both had our share of relationships until a while back we were both single and we wanted to go out with each other. We talked on the phone a lot, one time we even fell asleep on the phone together at like 3 in the morning. It was so special you know?. But she didn't know I knew she wanted to go out with me. A friend told me. But I didn't do anything about it. Didn't ask her out or nothing. The opportunity passed me by. So we like facebooked each other rarely up till now and we txted each other for a while the other day and planned to see each other at a 4th of July thing. We both are in a relationship. I know I would give up my girlfriend for her. But who knows about her and her boyfriend. She seems to like him judging from her Facebook updates and stuff. But life keeps pulling us back together and I don't know if she feels remotely the same or what. All I know is I don't want to lose her again if she is interested in me. So the final question is obviously, What Should I Do?