I was asked that question when I started therapy about 7 weeks ago, and I think that I have finally forgiven myself. I have always been my worst, toughest, harshest, critic, but I do think it is necessary to forgive ones self before the healing process can begin.
I started apologizing to my closest friends and family members as soon as my affair was exposed, and that helped lessen the load, so to speak. It started making it easier to forgive myself, and seek help for the issues that lead to the affair. (Still not sure what some of those issues are, but we'll find out)
The part I am having the hardest time forgiving myself for is the pain that I caused. I never meant to hurt my wife. My affair started off on line through a social networking site. The relationship started off with no ill intent, but gradually evolved into a full blown affair. For some dumb reason, I thought I could run that relationship alongside my marriage, (before the actual sexual encounter) and not have any connection.
It turns out that that wasnt the case, that the two were interfereing with each other. I didnt want to lose my marriage, but I couldnt just drop the affair until I was handed an ultimatum. Maybe it took standing on the edge, looking at all I was risking, for me to realize that it wasnt worth it!
I am fortunate enough to have been married to my best friend, and we are wading through the much and mire together. We are both in therapy along with couples therapy, and we take each day, one step at a time. This isnt an easy task, and I think we are both finding out some things about ourselves that we didnt know, and quite frankly is a bit uncomfortable...but it all has to be dealt with.