Sad to hear about the news man
Having been in similar situations, I think a lot of it is that it gets tougher and tougher to end a relationship the longer it goes.. So while you probably didn't do anything horrible, it was probably just the realization for her that you weren't quite what she had in mind for the rest of her life.... which probably isn't all that comforting for you, but certainly better than hearing about it AFTER proposing to her.
And I'm with Liss.. It's probably just too awkward to deal with being around you if you two had been in a relationship for so long.. It probably isn't anything personal and I honestly doubt it was some switch she flipped. People just have different ways of dealing with breakups and honestly I'm a lot like her in how I dealt with mine.
On the plus side it sounds like you're getting the rest of your life into gear so that's good :] I can't say I relate to the liberal arts thing as a career, but it is definitely something that is preferred (maybe too strong a word, but you get the idea) even in the medical field, because as you said it is likely to produce a more well-rounded person in the end.
I know I don't visit here much but just letting you know I wish you the best of luck :]