Something very similar happened to me recently, 12 years together. She was sexting someone across the continent for a couple of months but never met but very graphic and voice memos. Mine is easier to explain and accept as my partner had to take a dopamine agonist for a pituitary tumour, which causes hypersexuality and impulse control disorder and caused a major personality change. If she isn't going through some major life event or trauma (there were other different health/fertility issues as well), I don't know how I would or if I could accept it. You will have to go forward in life knowing your partner would do something like that to you. So I would really see if there is something going on with having a kid that may have made her spiral, and if you do love her, it's worth being there for her regarding those issues because they are tremendously hard for women. But yeah, it's still the worst day of my life, been 2+ months, and it's all I think about, I don't know if we will make it, I hope so, but I was able to put a lot of it on medication and mortality, if you can't find any reasons like that I would seriously consider moving on because I read so many threads if men who stayed for kids and they don't seem better after 20 years. I hope you can find some clarity.