You may share blame for the marriage having problems that weren't addressed or communicated, but you are in no way responsible for his relationship with the OP. That was a choise he made (i.e. how he chose to ignore problems, thought it was exciting, decided it was easier to communicate with the other person, a friendship that just kind of sprialed out of control, etc...). I don't care how bad your marriage was. That was his choise.
Your responsiblity is limited to your own relationship with him. If he turned to alcohol or drugs to deal with his problems would you be "responsible" for that? Do we really have that much control over our spouse's choises? Sometimes I try, but I really don't.
There is no arriving, ever. It is all a continual becoming.
