I'm so very sorry that you're going through this. With the differing and agonizing emotions I'm sure you feel as though you are somewhat crazy on this roller coaster. Please be assured you are not.
New Years day was only 12 days ago, and you said there were multiple days of discovery. I'm certain you are being too hard on yourself concerning your healing. There is no time frame of healing, each person is different as with the circumstances. The phases or stages may be the same and still unique to the situation and persons involved. I'm affraid I don't know of any short cuts for this, I do know it isn't healthy to skip parts of the healing process. Good days and bad days are expected, this is normal, including the imagery. In time the good will or could outweigh the bad.
How often has he seen a/his therapist? Why do you think his and her actions were 'compulsive'? Has the OW/your friend been confronted(in the sense it's not a secret anymore)? Can you speak with your husband openly with what you are experiencing? If so, is he validating you?
As FCL suggested, a marriage counselor would be of great benefit for you and your husband. And the other exchange that has Dr Becker-Phelps featured as an expert.
I've heard many times that the children were not informed, then later on wished they had been. For purposes to clear up any confusion or rifts, and there be a better understanding of things that arose from the situation. NOT AS VINDICTIVE!
As for the OW, your friend, for myself I'd let it be known that at this junction I have nothing to say to them. If there is any hope of repair for the friendship now is not the time. As for being in the same social circle, I might cut down the time I spend attending and if someone asks why we(friend & I) are not 'joined at the hip' I'd say we had already talked/spoken and/or that maybe I was feeling a bit under the weather. This is also an opportunity to broaden your circle. As for the trust being restored, that's as unique as the situation, circumstances and persons involved.
I do hope you feel better, or have more positive thoughts soon.