thinmint, welcome.
IF your gut is screaming at you there is a reason. I have always believed this, even when my H was in the throws of his A. I would try to deny it, but then I would always come back around.
From someone who has been there done that, there are a few things that I would strongly recommend. Things I did do, that helped me, things I should have done, or done sooner, to get my proof.
1. Quit asking him about it, quit bringing it up as much as possible. Get a keylogger on your "shared" computer ASAP. I would also do a in depth search for a second cell phone, we often refer to it as a burner phone here. It's an easy way for them to communicate without getting caught.
2. Spyware on his phone if you can. If he has a home office, I would put a VAR (voice activated recorder) in there.
3. See an Attorney ASAP. Find out what your rights are, what your situtation would look like if you would have to make the hard choices. The fear of the unknown tends to paralyze us, so find out as much as you can.
4. Go to your Dr, get STD tested, and talk to them about something for anxiety/depression if this whole thing is overwhelming, and we all know it is.
5. Be kind to yourself, and your kids. Put them and you first. He certainly isn't.
6. Think about what you really would want, and how things can play out. When you get your proof, and you confront, be ready for him to do everything, say anything, and make you feel crazy, all of that comes before they actually confess. If he confesses what will you want, figure out if you want to try to save your M, and if so how would that happen, what would you need from him for that to happen. Have a clear set outline of your needs for R if that's the path you want. Also know that even if he comes clean and wants to R that he will stumble, fall, break NC.
We all talk about the cheaters handbook, there isn't really a handbook per se, but there are certain things they ALL do and say during, and after the A. He is certainly doing his best to make you feel like the crazy one right now. Know that you aren't. Something is up. Keep reading here, the healing library has a ton of info, read up. It's over there on the left side of your screen. keep posting, keep telling yourself that you are worth more.