So sorry you are here miswestmom... my wife was on CL in a long term relationship with a 2x convicted child molester. They would look for others interested in swinger's lifestyle. Really quite disgusting. You have already received great advice from JustDandee.
You are on the brink of a severe emotional pain even more than you may have now... Once you have confirmation from him, it will be a waterfall of emotion and pain quite a bit more than you feel now. Somehow just seeing this stuff pales in comparison to the confrontation and the admission. Mine didn't tell the truth right away and I had to hack the information out of her.
Consider secretly recording your conversation with him when you do it.
You can place a voice activated recorder under the driver's seat of his vehicle... use velcro to affix it. Also can use GPS attached by magnet or enable google maps to track his location. Be sure to have control of all places he checks mail when you enable it because any new place you logon to his account and anytime you start the gps logging it will send an email.
Check bank statements for cash withdrawals and spending money at places that are not familiar.
If you have the money, hire a PI.
Not sure where you are in your relationship whether you'll stay with him or leave but certainly agree with seeing an attorney.
Any proof you get should be stored in copies and away from your home where he can't reach it.
Something to know about cheaters is that they are living a different life away from you. This life is giving them great satisfaction and for some, any threat to that causes either violence physically or mentally. The trust you have for your husband is gone, yet you have x years that you've known him. It is an imbalance because your mind says you know this guy but your logic says this is not the person you know. The lost trust cannot be regained, it can only be rebuilt from the ground up. That is hard to do because in normal relationships, you learn about someone as you begin to trust and vice versa.
I think in your heart, you know if you're going to stay with him... there may be certain boundaries that you have but after everything my wife did, I wanted to stay with her and I knew it the moment I found out. Your emotions and logic are going to play at each other through this regardless of what you choose. One moment you can't live without him and the next you want to wring his neck. This is a normal and healthy response... well not actually wringing his neck though!!
I think you should establish a boundary of what and how long you are willing to put up with this to get information and for him to stop. And you need to stick to those boundaries.
At a minimum...
He needs to come clean completely with you and tell him that. Trust cannot be rebuilt and we cannot move forward till I know everything. Because if any little tidbit comes out later, its going to throw us back to the beginning. Suggest maybe him journaling it on a timeline for you. If you need, schedule a lie detector test.
He needs to enable GPS tracking and provide his location to you voluntarily anytime he goes somewhere. This is not because you are that way, it is for him to rebuild accountability and trust to you.
He needs counseling whether that be pastoral or professional it must be someone EXPERIENCED in helping people through this.
Very important... every relationship has issues... its part of life. ANY marriage problem you two had did not cause this infidelity. We all choose how we respond and cheating is not a valid response. Separate the two... BUT at some point you both will need to work on marriage problems so your relationship sees forward progression.
Like you and the rollercoaster of pain you're going to go through, he will go through that too. He's addicted to his behavior because it makes him feel good but he "should" feel bad about it and avoid you or overreact in certain ways. It would be likened to a cocaine addiction. No matter how good of a wife you present yourself as, the addict will still go for the drug. Don't fall into the lie of trying to make yourself more "appealing" to keep him out of cheating. You need to be strong in this and it may be hard.
I wish the best for you and for him to stop his selfish behavior.