I understand why you are so hurt, even though he didn't follow through... the intent was there which is almost as painful as the act. We are all but humans though, and as much as you hurt... if you love him - if he is a good man outside of this incident and makes you feel loved... its worth working out.
He is going to have to go out of his way right now to make you feel secure, he should know that is the price to pay for his actions... in time you will be able to feel better I hope. He needs to cut contact off with this girl all together and that will be his first act of helping you to feel more secure.
He should know there will be consequences if he does contact her again, that he's already broken your heart once and you forgave... but if he does it again it may be too damaged to mend.
And then you have to let go. Holding on to the hurt will only hurt you. I know the ache in your stomach that you feel at the thought of it all, but you can't focus on that... you have to get your mind off it and use this opportunity to spring some new life into your marriage.
Sometimes change only comes from drastic and near catastrophic situations. This may be what he needed to see how much he doesn't want to lose you, how close he could have come to ruining his life. Hopefully it was an eye opening experience for him and that he gets his priorties in order.
For now, write how you feel, say, don't bottle, experience. Letting go, getting it out of your system through tears, through thinking all the love you have had with this man and how it means more to you than anything he almost did but didn't.