if she's just a friend is she someone he's spoken about before to you? No, thought not.
This bit is so hard, but it will get better.
I remember being so angry when I found out my ex had had an affair. I have never been in a fight ever, I'm a pacifist FFS! But I went looking for her as I wanted to hurt her
.
Boy am I glad I didn't find her! How totally wrong that would have been. I can look back on that time now with my head held high. I didn't do anything wrong. Had we got into a fight or even a verbal confrontation it would have been ugly and I would have been seen as a nutter!
And do you know what - although i have zero respect for her, and never will have - actually she did me a favour - I'm so glad I'm shot of my ex now. I'm so much times better off without him. My DP now is lovely, and I am in control of my own life.
Your exH didn't treat you well at all. Once you get past this bit you will get to a point where you are glad to be rid of him, I promise.
You need a plan to get through these first weeks. Be kind to yourself.
Could you go visit a friend for a few days maybe, just to get away?