I am very sorry to hear the place that you are at right now and can only imagine the pain and hurt you must feel.
I do not agree with the posts that have been posted here above because I feel once you step outside of the mariage and cheat you are breaking every single vow that you two ever made.
You are a good person and have tried to work things out in the past and that must have taken a lot, but you also have to remember that you need respect and honor from your partner not for him to be going off and cheating.
It sounds like you have raised a good loving family, and will always have the happy times together, but please do not allow yourself to be placed in second place by a guy that wants to have all the cake and eat it.
Do you really want to stay with a guy sleeping around behind your back ?
Do you want to have the risk of him bringing things to you, such as STI's etc ?
It is hard to do because reading your post you are feeling very weak at present, but you need to be strong and stand up to this and sort it out once and for all.