Women are very selfish when it comes to those things. It is very possible that she wants your attention, yet will not dump her boyfriend. If she's developing feelings towards you, it means she's realizing she likes you more, or she just realized she has a better offer in front of her and could upgrade. With that being said, it doesn't mean she'll dump him and let you in. Why? Because it would make too much sense to you and I, but not for women, those illogical creatures who somehow get more of a kick to find themselves in situations of drama, and if not, find a way to create it.
The best approach seems to be to withdraw yourself from this situation. Don't worry about her guilt, that is her problem. Your job is to spike up her interest level. Most of us have probably been there, and know that for the most part, at best, it will take a lot of time. Human beings want what they cannot have, or what is hard to get, or rare commodity. By withdrawin', she will miss you and your value will definitely go up. Since she already has thoughts of how things would be with you instead of with him, this might help your chances. Off course, the best thing is to stay away and not worry about it. But we always seem to want to go down swingin'. So if you must:
You can ask her in a c/f way
"are you feeling guilty you like me more?"
or say something like:
"You're feeling guilty, huh? There must be a reason why you feel that way, would you mind telling me?"
Just listen to her response, don't interrupt her, don't complete sentences, let her totally tell you what's on her mind. Pay more attention to her reaction, and how she says it and her body language, wich will tell you a lot more.
