I can relate with the difficulty of not having closure on relationships, but quite honestly, it's just not even worth it sometimes. I think she would only blame you and make you feel worse. I don't know why my last relationship ended and some days it bothers me but knowing would bother me even more. On top of how it would bother me, I think it would all be a pile of BS anyway, just something to suffice. Once, I pushed an ex of mine for answers and he said some horribly hurtful things. Years later, I still remembered those things. Recently he told me that what he said wasn't the truth at all, it was just what he was feeling at that time. He wants to be friends (like your ex) but I can really only handle minimal conversation with him because it's no worth it. My recent ex... well, I will NEVER speak to her again for my own stability & sanity.
It seems your ex jumps around and didn't appreciate what she had in front of her. Like you, I gave everything I had, sacrificing important things along the way. Rejection sucks, no other way to put it. But like so many have told me, it isn't my fault. I just leave it at that point. I will not have any regrets because I darn sure tried my hardest, as I'm sure you did too. It just means that someone else is better!
You seem to really have your life in order and I'm impressed with your interest in tbe business world. I graduated with a business degree and hope to put it to great use one day. Keep your head up... you will still be really bothered some days and you will wonder how she can be "so happy"... but in the end, it doesn't matter at all. And she probably isn't very happy. Not truly happy.
Hope that helps!