Sorry you find yourself here MoGirl but you have come to the right place to help you get through this. My H (XH now) was also leading a double life...for at least five years. I only know about the last OW who he is marrying in July and asked her prior to being divorced. He only met her two years ago so I am sure there were others that he will never admit to. Like you, when I found out, it was the WORST pain I ever felt. I’m not going to lie to you. It lasts awhile and it sux beyond belief. But...eventually the pain fades as you find your feet again and start to adjust to your new normal. There IS life after betrayal and it can be a good one if you make it that way. I am living proof. You can and will get through this. I promise you!!! You will get to the other side and even find yourself feeling glad he is gone because living in the light is always preferable to living in the dark. My XH is a shell of a person. He is clinging to this new R like a lifeline because it is all he has left. He has damaged all of his other relationships. He has likely convinced himself that he hasn’t but I know different.
Re: this 21 year old. She’s already been married? That says all you need to know right there. Let your H go. I give this new relationship two years...less if they manage to find a way to live together. She is a child. Once the hormones and chemicals fade, he will find himself in a world of hurt. This R has about a 1% chance of going anywhere so feel confident that his pain is coming. And when it hits, you will be long over it and wondering what it is you ever saw in him in the first place. Anyone capable of cheating on his wife and family is not worthy of you. Let her have him.
Gotta get to work but just wanted to say hello and give you some support. I know what you are feeling right now. It is awful. Have faith... it will get better if you take the focus off of him and what he is doing and focus on yourself. (((HUGS)))