I've been through almost the exact same thing. In fact, aside from the kids, and the gender difference your story and mine are very much the same. I know there are days when you picture your husband and the other woman, and that you can't get that picture, that mental movie out of your mind. On that, I have two truths for you:
First, you will never get those pictures or those thoughts completely out of your mind. It just doesn't work that way, no matter what.
Second, what you envision is almost certainly worse than the reality. We make those movies in our heads, but we exaggerate them, partially to justify the pain and anger we feel, and partially to punish ourselves for what we perceive as our own failures, but which are actually the failures of our spouses.
Neither of these truths make you feel better, but I wanted you to know that you are most certainly not alone here. Right now, you are at a crossroads. On one side, if you leave him, and if you get some treatment and counseling to help you deal with the loss of your marriage, there is a possibility that you might meet someone who is great and who cares for you and your kids. Of course, you also might not meet this great guy, this is a gamble. The only sure thing is that this guy won't be able to hurt you again.
Down the other path, you work to save your marriage, and forgive his affair. Maybe you and he, again with treatment and counseling, maybe you have a long and happy marriage for the rest of your lives. Maybe he never does it again. On the other hand, maybe he does, and you face the same crossroads later on in life. Or maybe you accept this because of your kids. The only constant here is that the trust you once had for this man is gone. You will always be a little suspicious, and this will cause actress on you and your marriage.I don't know which is the right path for you. I chose to forgive, and she wound up leaving anyway. It screwed me up badly, and I'm still trying to find all the pieces to put myself back together. I hope better for you.