I know it's not nice to hear, but your chances of this succeeding are so remote its virtually a guarantee for pain and receiving a blow to the head when you crash. Just go with me and read my view here, because I am sure you are a decent bloke and people like me are taking time to write this stuff to actually provide some help/advice to you. Its not a rant to berate, its to arm you with objective info you may not be able to see because you're influenced by this girl's attractiveness. Never met her obviously but let me bet you have good taste when spotting a sexy girl and she is sexy as hell to you.
Here's the list of the grim (yes I accept not nice to accept) scenario's:
1) As mentioned above, she is looking for a parachute to break her fall.
It can be very scary for someone ending a relationship with nowhere to go. If you're an attractive girl, why not use another guy as a parachute if he is so openly and cheaply (read "free") offering this service? I have a theory about what happens to a parachute once landed however. It's scrunched up into a pack and put away. It's no longer needed once the "jumpee" feels safely landed on to solid ground and satisfied they can get up and walk on their own two feet.
2) She is receiving no attention from the actual boyfriend and is tantilised by feeling attractive to another guy (you). It such a common phenomena that that chicks become attracted to attention from men. But consider this - they are not necessarily attracted to the guy, they are attracted to the attention they are receiving. They need attention like blokes need constant ego stroking.
Remember this formula for life:
Women's vice/insecurity = attention.
Mens vice/insecurity = ego stroke.
Ration out the supply of these two commodities to both sexes and you would commonly have control over most people. As for the attractive lady however, once they receive enough of this attention they usually shut that tap off with utter ease and efficiency.
3) She does like you and wants to be physical with you, but she also like's her BF, or at least likes what he once was and doesn't want to end with him in the hope that he may go back to being the suave,charming and attentive dude that she went weak at the knees for in the early days. She wants that buzz back with him as he amazed her before and she burns an outside hope it/he might come back. She therefore, (again as someone mentioned above) likes this triangular approach because she gets to have both of you and choose who and when. To illustrate my point and without trying to sound patronising or childish, most guys would take both a Ferrari and an offroad SUV if allowed to have both. Most guitarists will take both Fender and Gibson if given. Its not that they like one and not another, but why not have both if given?
Now would you like to be a car only driven sometimes? This is the ultimate question. If you'r answer is "yes I want to be driven a few times a week and then be free to sit in the garage the rest of my time and do whatever I feel like", then go for it, ask her out again. If your answer is "no I want to be the exclusive automobile that is driven", then you need to consider the odds are not good at all. Its truly a choice of what you want out of it. Most things in life are choices, we often just dont like the options we can choose from.
Personally if I were you, I would call her once, flirt big time in a warm way, then vanish totally. I mean totally vanish. Maybe contact her once every 6 months to test how things are going if you have to, but I would be so unavailable that she can decide to escalate if she wants to. Really she will decide, not you. Regardless of if there are 500 posts to your thread here and everybody has a view, she will still decide irrespective of 500 different sources of advice.
My own personal experience: - women spend long periods of time in the "I want to be break up with my boyfriend" or "I'm about to break up with my boyfriend" mode. This mode can extend over a period of decades. Hanging on the side line during this period is truly a waste of your time but worse, is a slander on your self respect.
