I wonder if professional counseling might help, although it will certainly be me who goes, not my wife. I guess the worst part of this is that I am not convinced that she regrets it. I am completely at a loss for what to do. Different surveys come up with different figures, so Delilah's not going to try to guess-timate here. Just assume that affairs happen, and many couples have to deal with the consequences.
Some couples are able to put their relationship back on track after learning about an affair.
For other couples, the news of an affair has such harmful results that the relationship can't survive. It can just be too hard to get over the sense of breached trust and betrayal that occurred. It seems like you and your wife have personal reasons for staying together. Given that, it's clear that you haven't recuperated from the pain of seeing your wife with your best friend, nor have you been able to re-establish the trust that existed earlier in your marriage. Professional counseling is an excellent idea.