All I know is this. I was in the same situation. I was young ,had a lot of personal problems which spilled over into my marriage.
She wound up having an affair and used the same logic. I was lonely , etc......
Bottom line is guys look for girls that are in a troubled marriage or whatever and take advantage of the situation.

My situation was reverse from yours though. You cheated first.
All I know is two wrongs don't make a right.
Bottom line is this. you will never trust him again and he will never trust you.
Trust is the foundation of a healthy relationship.
17 yrs. is a lot to throw away.

I wound up trying to make it work for many, many , years. Sometimes when something gets broken it can't be fixed.
I wasted my whole life on a marriage based on a lie. Almost 40 yrs. of hell.
I wound up doing now what I should have done a very long time ago. Get divorced.
The only hope I can see is if your both willing to go to individual and couples therapy.
I would try that before a divorce. I don't see any other way it can be "fixed".

Best to you,
CB