Determined,
You bring up a good point. The only thing I can say is if this girl and her boyfriend were in such a strong/steady relationship to begin with, then I wouldn't stand a chance to begin with.
As for your point about if she can break up w/ her live-in boyfriend, then she could break up with me in the future, there's always that risk. But like most things in life, I view it has nothing ventured, nothing gained. There are no sure certainties in life, and as long as I keep my expectations grounded in reality (i.e. have no expecatiations), I can live with the risk.
I'm sorry to hear that your girlfriend left you, but at least you found out before you reached a deeper level of commitment (marriage).