Oh sweetheart. I know the pain you are in. You thought it was an old wound but his truthful confession tore the scab off. You are now dealing with this as a new fresh wound. All I can say is the mind movies will subside with time.
You say you love him more than you ever thought possible. To me that is a reason to try to work things out with him. He needs to be supportive of you and not try to shut you up about it. He needs to own his mistake (which he seems to be doing thus far) but most importantly he needs to have 100% no contact with her. He needs to work on building your trust again. A few things you can do together is make a list of trust building behaviors that are meaningful to you.
Maybe something like, if she contacts him he tells you right away and does not respond to her or he will call you asap if he is going to be late at work anything that means he is trying. But most importantly remember you are not alone.
You say you love him more than you ever thought possible. To me that is a reason to try to work things out with him. He needs to be supportive of you and not try to shut you up about it. He needs to own his mistake (which he seems to be doing thus far) but most importantly he needs to have 100% no contact with her. He needs to work on building your trust again. A few things you can do together is make a list of trust building behaviors that are meaningful to you.
Maybe something like, if she contacts him he tells you right away and does not respond to her or he will call you asap if he is going to be late at work anything that means he is trying. But most importantly remember you are not alone.