He is still in a fog. He is not dealing with what he has done to you, your marriage or your family.
Nothing he has with this 21 year old is based on reality. It is based on lies and deceit. It is easier to wonder about her than it is to deal with the mess he has created. All that was fun, exciting and new. Rainbows and sunshine not mortgages and kids, and real life.
he is assuming that he has damaged the marriage too bad and maybe doesnt think that any effort to save it will work
That is a cop out and a coward's response.
He spent how many months dedicating himself to a lie? He can't/won't put the hard work in to working on his marriage?
It takes on average 2-5 years to heal from an affair. It is a long road but you can make it if he is willing to do the hard work and stand by you through your pain and despair. Too bad he can't stand seeing you go through the pain. He caused this pain, he needs to stick around and administer CPR at this point. Action and accountability.
He needs IC for himself to figure out why he allowed himself to cheat.
Hang in there and know that you are better than being an option.
Read the 180 and try to take back some of the power.
Good luck. You will be okay. I promise you will.