Getting over OG, whew! That is a hard one. For me what worked was understanding the damage that my holding onto thoughts and memories of OG did to my marriage. I was able to finally stop clinging to the memories and hopes by understanding the danger there. It was like playing with matches. When my house caught fire I figured out that it wasn't really fun.
My sitch differed from yours in that my husband finally took my cue and lifted his blinders and found a girlfriend. So far as I know, your husband didn't. So I can be angry at my husband for having an affair. But I can't fail to look back at myself and say, yes, but you had an affair too. You had a love relationship, even if you hardly ever saw that person, of 25 years. And by holding OG in your mind, fantasizing about him when you had sex with your husband, and in a lot of other ways making that relationship more important than this one, you undermined the marriage.
So if you are not building, then you are tearing down. I have stopped tearing down my marriage. I am building it. And I don't have any contact with OG anymore. When I think of him, I think of how destructive that thought is. And I put it aside.