Please help me make a decision.?

A year ago i found out that my husband had been having an affair of and on for over 5 years. I was devastated. I told him to leave, he begged and pleaded to give him another chance. I was so increadible hurt and in a state of shock, as we had been together for 18 years i had never suspected. My children were happy dad was back and he seemed to want to do anything i wanted him to do. He has gone through, individual therapy, couples therapy, marriage retreats, been transparent with me about his wherabouts, mostly he has been loving and attentive to me like never before. The problem is I just cant seem to forgive him and get the ugly images out of my head!! I am so full of anger and rage when i remember all the deciet and lies!! ually i cant stop thinking about him being with her and how he must have lusted and desired her even if didnt love her, it still bothers me that he would lust and desire someone else. When i ask him Why? he says he didnt know what he had, and most of all he didnt know himself. I try to believe him and he is showing me he wants to be a better husband but i just dont believe him. Sometimes i will rant and rave and cuss him out for hours and he will sit there and say nothing. I am just so lost, my therapist says i have PTSD and all the ugly other emotions that go with a betrayal. The question is will i ever be able to forgive and forget or is this marriage over? Its been a year and i am still very angry. It is very sad, because i always wanted him to love me and show me how much he cared, and now that he is doing it, i dont believe any of it, is it too late?