And I am saying this because I was where you were. I had an infant and I just wanted him to not leave me. I would have let him back in unchanged at first. Hell, I begged him to come back. I screamed, I told him how awful he was for doing this and I tried ot guilt him back in, I tried everything.
Hind-sight vision is 20/20. Looking back, if he dumped OW and came back home, but nothing changed, well, I would have suffered longer, and I would have been exactly where I am now 9 years later. Divorced.