I am with xdude, could be the lack of real daily interaction had something to do with building up your feelings to such a level. If you really consider that point, you would have a very concentrated formula of true relationship reality. She may or may not have been a serial cheater, doubtful considering the excessive amounts of attention she lavished on you. I wouldn't waste too much time trying to dissect HER reasons and focus that energy on YOUR reasons. She sounds to have more of a need to mother complex brought on by the aging of her family. While it is understandable she would do anything to save that unit on their behalf and her own it makes no true difference to your outcome and what needs to change for you to move beyond this and become healthy relationship material.
You ended it remember? That part is done and now you need to allow yourself some time to heal.
You exposed her for a reason so as to either hurt her or to end it or both maybe. You probably accomplished both because it doesn't appear she tried ending it by what you have stated. You are free to explore why it happened within you and explore how you can avoid those pitfalls in the future. Give yourself a break and maybe her too. You probably will never get the answers you want on her part as it sounds like you wouldn't believe them even if she were able to tell you. She may not have had malice in her heart and just gotten in too deep for whatever reason. Still don't see how that is going to help you heal by knowing if this reason or that reason were the purpose on her part. Are you in counselling? And if so, how much money are you spending to figure her out in your counselling, money in my opinion better spent figuring you out and healing past this. You sound like a decent person who just got involved with someone who could not be for you what you were able to be for her. You may also be missing the friendship and having to mourn that loss as well?