I suspect after such a long relationship and by the sound of it one in which she allowed/was compelled to be subsumed under the BF's personality that she either by her own genetic make up or his conditioning has to have a stable thread running through her life that she can anchor her self worth onto. That's you right now and is something that you can give yourself a pat on the back for as there have clearly been other people she's had infatuations for and cast aside quickly, but you're still there and in her eyes (I suspect) gaining an awful lot of brownie points by not treating her as some quick and easy shag.
What she says does sound positive for your hopes and you two clearly need to resolve this to maintain your own sanities in the long term, You aren't married, have no solid emotional commitements to others, so are into far safer territory than many others who find themselves in a similar emotional area arising from a close and supportive friendship.
Ask.
Yes you may lose the friendship if its the wrong thing to do and it will hurt like hell, but you have built yourself up psychologially to a place where you will survive. look at this thread, from what you say across it you've grown from a bit of a mess into someone that can willingly open themselves up to pretty intense and intimate scrutiny - dunno if you've opened up to friends/family in the real world or if we're your safety valve before that leap of faith, but it's still a big thing to be as candid as you've been. She too has grown a lot by the sounds of it from knowing you and whilst you'll worry about her if it doesn't work, she is a big girl, has shown herself that she is still able be physically desireable** (to the fly by nights) and a long term friend/relationship (to you - although I suspect you find her 10x more desireable than any of the guys she's dated) and will be able to find her own way through whatever emotions are going on that have got you two to here.
**yes everyone I know but in her mind she's proving she's still got it after a long time off the scene and a significant rejection.
I do suspect however if it does go the way you hope it will go fast.
best of luck