You speak so eloquently about what happened and it's all reasoned out, the only problem is that when emotions are involved there is sometimes little to no reasoning. Your former g/f understood you and all you had told her for years, to the best of her ability, but she didn't live it, therefore complete understanding is impossible. I think it was Daisy that said if the incident you described was the proverbial straw that broke the camel's back, then I get it, but if not, then there is more at work here.
If a partner shows "weakness," it's a tremendous leap of faith. If the other partner sees that weakness as a fatal flaw then the whole relationship has been built on a falsehood. The strong one and the weak one come out in all of us sometimes and in a long time relationship, they change hands many times over the years. I could never be the strong one ALL of the time. I think we all need someone to hold us up every now and then.
I'm sorry for both of you.
If you fall down 10 times, Stand up 11.