I know you don't want to hear this, but you are so lucky that you are not the one that is married to this guy.
After my ex boyfriend cheated on me for years, he finally got someone pregnant. She is now five months pregnant and he still calls me and leaves messages on my phone behind her back ("I love you blah blah).
I am SOOO much happier without him. It hurt at first, big time. I couldn't eat or sleep, I know what you're going through. But now my life is so much better, so much more peaceful, and I just keep thinking, Thank GOD I didn't have a child with him.
Don't wait for him to leave her, please. Find someone who will be faithful to you. You're worth it.
You'll feel better, I promise. It just takes time, and not as long as you think.
Keep yourself busy and spend lots of time with your friends.
Watch funny movies.
Eat healthy food and exercise.
It'll get better.