Sorry you are here...but I'm also GLAD!

You can repair this marriage!
First thing you need to do is tell the OM g/f soon to be wife that he has been cheating on her since the day they met!! Do you honestly want to put her life in some man's life when all he will do is cheat on her? PLEASE!! Do HER a favor and let her know that her fiance is not the right man!
If you truly want to end ALL contact with this OM, then change your # or send him a NC letter. Block his FB account, change your email, etc...so he can't get a hold of you. Maybe even move out of town!
The next step to do now to fix your marriage is start reading everything and anything on this site. There is a lot of information here and a lot of people that are willing to help you out

Read the book "surviving and affair", and "her needs, his needs" together and do the questionnaire.
Tell your husband EVERYTHING!! I mean EVERYTHING!! I know it is scary but you need to be open and honest with your husband, if you are afraid what he might do then have someone there so he wont do anything rash to you.
Have you told anyone about your A? If not then I suggest you do that, expose to everyone you know and let them know what has been going on in the marriage, don't talk negative about your husband just what YOU did

Also your kids needs to know what you have done as well, they can not be kept in the dark, if you think you can't, just to "protect" them then all you are doing is teaching them to hide and lie, is that what you want?
There are plenty of things to do, but let's start with these shall we?
GOOD LUCK!