I am so sorry you are in this situation.

I hope you are taking care of yourself.
Don't leave right away. He should be the one to go, if anyone leaves. Give it some time to figure out what you want. But don't let him sleep in your bed, assuming you both stay in the house. (This is what we did as we have kids ages 14, 12 and 4, and didn't want to disrupt their lives.)
I can't tell you what to do. I can only tell you what I did.
I talked to an attorney, a counselor and a few very close friends before confronting him. I had a thick envelope of evidence. I invited him to lunch on his birthday -

- and busted him. I told him I was giving him a one-time chance to fix things or get the eff out, that day. I was prepared to take him to the courthouse to sign divorce papers.
He wanted to save the marriage. I gave him a list of my requirements, such as handing over all passwords, being accountable any time he's away from home, not being alone with other women (even friends), and getting a new job (he and the other woman work in the same building). I fudged on that one though since his office is moving to a new building shortly, and he does a very specific type of work that makes it diifficult to get a new job. I demanded all info about the affair and told him that if he lied about any of it, I'd be done. Stuff like that.
Get what you want out of the situation. If you want to make it work, lay down the law. If he balks, give serious consideration to leaving.
Copy all tax documents for as far back as you can find them. Also copy all papers that document any joint assets, including your home. Sometimes people get nasty and start hoarding joint assets and money if they think the spouse is leaving. This can all be reclaimed but it does take time. Have a credit card in your own name or some cash set aside in case your spouse freaks out and hoards your money.
I hope it goes as smoothly as possible.
I also sent a memo to the OW in my situation, at the same time I met with him, telling her the jig was up and that she needed to tell her husband before I did. Then I followed thru. If your husband's mistress is married, that person has to know. Affairs thrive in secrecy and if it's going to stop, everyone needs to know the score.
Good luck to you.