would be nice to know why did they break up did she hit her daughter or something
It's not explicitly told, but I think it has to do something about balancing both work and family life. We see her arguing about a gift or merch her kid asked her to get, but she refused (she got her a similar one before, that's the constant scene of a flashback we get where she is angry with the daughter) and at one point she raised her hand to "stop making a scene". There was another scene, iirc, about an echoing thought of"not being prepared to handle a child while working"? I may be wrong, after all. It's also implied that her ex was a workaholic (and she even insults him as "an idiot").
Like. I don't think it's enough for a divorce. But, culturally speaking, we know how Japanese society and their (traditional) nuclear families tend to be. Maybe their marriage wasn't getting good, they were both busy with work, couldn't raise their child "properly" together and I would understand that she maybe hitting their child was the last straw that broke the camel's back.