Oh OP, i have been there too. My ex of 20+ years, always other women on the fringes of our relationship but not enough to end things. Until 6 years ago he tried to get a mutual friend into bed, he had always fancied her and i found out before anything happened.
It broke me but we stayed together. 3 years later i ended it as i couldnt cope anymore, there was other things but the main one was that the trust was gone.
When he moved out i felt relief and i knew i did the right thing. After 2 months he moved in with the woman who he tried to have the affair with.
it broke my heart but i felt i had no right to feel that as i ended things, and it also dragged up the feelings from before when i had discovered things.
It really did knock me for six and i mourned the relationship we had, the future i thought we were going to have and for the man he used to be.
Everytime i felt overwhelmed and felt i couldnt cope i just thought to myself "This will pass, this feeling will pass" and it always did. The sadness would hit like a thunderbolt but the feeling always passed.
My favourite mantra was "Sadness flies away on the wings of time"
I am now with someone else who is lovely and i am head over heels for him.
Give yourself time to grieve and expect your feelings to change day by day, hour by hour even.