P.P.
Of course, this is totally your decision. Think about, do you still love him, can this marriage be saved, can you ever trust him again? You do have to confront him about it, when and how is up to you. This will probably be one of the hardest conversations that you have ever had to have. You could just leave the notes laying on the bed, and maybe this would make him talk to you instead of you bringing it up. Just whatever you do, make sure when you start the conversation, that the kids are not around, this will be a very emotional conversation and something they don't need to hear.
My husband and I went through this twice, and both times we said this was it, we're through and that he or I loved someone else, but in the end we decided that what we needed was the excitement of being with someone new, not that other person. We talked it out and decided that we could make it work, by working on our relationship instead of the new one. We started acting like we were dating again, flowers, NOTES to each other, cards and the whole new love thing. I am happy to say that we have now been together a total of 25 years. I tell people we'be been together 25 years, 20 good years, 2 great years, and 3 total hell years. I hope this helps and if you need to talk, just email me back.. I have a pretty good ear. Good Luck and I hope everything works out the way you want.