I know this very well. It seems like I'm an unlucky wanderer, I'm going from relationship to relationship, hoping that this time I won't be disappointed, or at least not hurt, but it never happens. If I have doubts about the other person, I try to negate them, because I think it can't be true there are only disappointing and neglegting people out there, so it must be my own imagination/perception. But then the other person does something which finally makes it completely un-denyable they're as bad as I percieved them. It might be MI which makes experience like that, but IDK, that's what happens as far as I can tell.
...you open your heart to someone, full of hope, and then you get disappointed...