How long was the affair?
Was this someone you knew personally?
How has your marriage been since?
Was this the only time he cheated?
It could be possible that you cannot get over it on your own. You may have to look into speaking with a therapist, who can help you develop coping skills to deal with your anger over the situation. Yes, your husband made a mistake, he did something he should have never done, but the fact is, he's human. As humans we all make mistakes, and 1 1/2 years is a long time to make him pay for that mistake. Yes, it's something you have to live with, especially when you see her, but you have to make a conscious decision to allow yourself to forgive and get your life back. If you allow his mistake to control your life, you're actually causing more damage to yourself than he caused to you.
Try and remember that what he did had nothing to do with you. Even if he were to say that the reason he did it was because of you, if he didn't try and work those issues out between the two of you, then it still has to do with him going outside of the marriage instead of putting that time into the marriage. Once you can let go of the fact that it was his problem (yes it happened to you, but he is the one who made the decision), you can let go of any guilt you feel inside for the situation, and you can ask yourself, can I forgive him for this mistake. Do you love him enough to forgive him? Do you want to stay with him?
If you love him, still want to be with him and want to stay with him, then you have to look him in the eye, forgive him and let it go. Talk to him about it, let him know that you want to forgive him and you want to let this go. It sounds like he really is a good guy who made a mistake, and that he's done everything to try and make things right. Once you understand that your anger, distrust and pain, could actually go very far towards making him repeat the mistake, or start to get angry towards you, then it may help to think about that and allow yourself to forgive.
I welcome your thoughts, let me know if you want to talk more