HBM
I hope you are feeling OK.
It was less than 48 hours ago that this all started. This new situation you find yourself in is very new and it hasn't stopped unfolding yet I don't think.
Now isn't the time, as others have said, for deciding anything definite. It's the time for getting through the days as best you can, and letting what you know sink in so you can decide how you feel about it when your emotions aren't swinging wildly from anger to sadness to confusion to hurt.
It seems to me that the thought of being without your H permanently is too painful for you right now. New pieces of information seem to be set up as lines in the sand "now he has gone too far", "no NOW he has definitely gone too far and there's no way back". But each time it seems, to me at least, as though you're saying that to punish him, but it's hurting you as well.
There are women on here who have come back from worse than this with their DHs and who have better relationships now than they ever had. It takes strength and courage, but it is possible in some situations.
I'm not saying you should (or could) forgive in the future. There is still too much you don't know - about what really happened, about how he feels about her now, about how you will feel about him as time goes on.
If I were in your situation I would incline towards being definite very early about what I was going to do, but I think I would just make life harder for myself by doing so. I think the smart thing here is to play a long game and remember that you're really at the very start of it.
Three days ago you knew how your future would be and now you don't. But living with that uncertainty (and the possibilities it contains) seems better than replacing it with a new certainty (that you will divorce him) that you aren't ready for and that may well not come to pass.
I don't know if this will help, but I hope you're doing OK.