I'm sorry to hear that RSG, it is the toughest thing in the world to hear, just know that you will get through this. Your going to likely start crying again, and question everything that she has done and wonder how she could ever do this to you, where did your loving wife go... These are all normal things to feel, they hurt a lot, but they are normal.
Be careful with ultimatums, you don't want to issue ultimatums when your emotional, because your gonna feel different about it in 5 mins, 4 hours, or the next day. I promise you that what you want will change constantly.
Work on boundaries, not having OM around your S is a good place to start, but you need to create a true boundary with a cost for not observing that boundary. Re-read the boundaries link cadet gave you on page 1.
As for the OM, don't bring him up, in fact try not to even think about him, he doesn't matter, because if not him it would be someone else. You need to focus on you and your S. Go back and read Sandis rules, that's what you need to be doing now, it's more important than ever that you detach and focus on you. The A has to stop before your WW will want anything to do with you.
I'm so sorry that this is happening to you, I pray for you to have the strength to become a better man for yourself. And if I can offer a glimmer of hope, your young and have plenty of life left in front of you. I didn't even meet my W until I was your age, and if you become a better man now, you will have a wonderful future with someone, wether it be your current W or someone else.
M - 9 1/2 years 5/5/16 - Bomb drop - 3 week EA 10/31/16 - We sold house 01/10/18 - D Finalized