I must say, even 1 1/4 years out, it is still difficult to deal with. Even if you strongly believe, as I did, that my spouse would never have an affair, you know how affairs typically happen, and who they typically involve, so when something like this happens, you really are just so unprepared. Even though my mind "went there" during the whole thing, (i.e. the idea of my wife having a physical affair with her best friend), it is easy to dismiss it because it does sound preposterous, and can be chalked up to red-blooded, American male thinking about having a threesome. So I spent considerable amount of time in therapy while this was happening convincing myself that a PA wasn't the case. I wanted my wife to be happy and I could easily see how happy she was when we were together with them so I chose to ignore some of the signs.
The truth of the situation was, I was being codependent on my wife and she and the OW were codependent on each other. One of the real lightbulb moments for me was my therapist explaining the difference between codependency and inter-dependency. I think my wife and I now in a place of inter-dependency and that is certainly a much better place to be. Another concept that was brought up was relational idolatry or putting the relationship as the highest priority in your life. My wife had put her relationship with the OW as the highest priority in her life. I don't know if you are a Christian or connected to a faith community but it can be both a blessing and a curse but the material coming out of some Christian counselors has been helpful to me.
Some books that were helpful to me:
Conquering Codependency Telling Yourself the Truth TrueFaced
