Labug, I struggle with the same feelings. I think part of it is because I gave my heart to this person, I let myself be so vulnerable, I trusted that I would be loved unconditionally, through good and bad, I thought he was strong and honest and had integrity. And I can't believe I chose a partner so poorly. It's hard to come to terms with how delusional I seemed to have been about him and our relationship. I wonder how I can trust my instinct about another person.
(Overly dramatic and irrational, but that is something I'm working through.)