You don’t have to confront him if you’re not ready for that yet. Depending on your financial situation that can get messy, do you have any children with him? How much do you do together, e.g. weekends together etc?
You can draw a line and decide for yourself that this is not how you want to live going forward. And start detaching emotionally. Practice radical acceptance, don’t sugarcoat things, don’t cling to hope he’ll change. Because he won’t. Well, one day when he’s older, impotent and unwell, he will probably want you to look after him etc but imagine trying muster compassion for someone who has caused so much pain to you through the years and who you actually resent. Stuff that.
Start reading about what you need for divorce and gather all docs you will need, he’s doing you a favour by being away half the week, you have time and space to see a lawyer and prep without him knowing or interfering.That’s a good position to be in.
Instead of considering a PI or trying to catch him in the act, start imagining what will change for the better when you don’t have this current weight on your shoulders. Things you would like to do with your life, your path, away from this depressing mess. Start paving that path in your head.
My ex didn’t cheat (that i know of) but there were strip clubs and porn, i stayed for many years because we had small kids (expensive) and it wasn’t bad enough to leave. Because of what was going on though, i was completely emotionally detached by the time i decided to go ahead with divorce. I’ve gone through all the grieving of our relationship whilst we were still ‘together’. There was no post divorce grief, adjustment to new situation- yes, but no grief or sadness, i was long done.
There are happier days ahead of you, you just need to chose them💐