Forgetting your wedding anniversary is not good, but it's also not an unfixable mistake if you take it seriously. But I do think it seemed like the husband was implying she quit her job to take care of their child. I don't think long-term parental leave with guaranteed employment is a thing in Japan, especially since it's very cultural for women to quit working when they marry and have children.
Lots of people here assume that she got divorced on a whim, but fundamental differences in values and life goals is a very valid reason to divorce. If the guy needs her to quit working to be happy in the relationship, and she doesn't want that, then that qualifies as a fundamental difference in values. They could try figuring out some kind of compromise in couple's counseling (if such a thing exists in Japan), but divorce seems like a plausible and reasonable outcome.
I'm actually quite positively surprised that a manga has a sympathetic female character that actually expresses that she doesn't want to be a housewife (especially after trying it out). In most manga (that portray women of working age), it's like it's a foregone conclusion that women quit work when they get married. She somewhat questions her decision, saying she didn't consider her husband's feelings, but I'm not sure what she was supposed to have done if she did. I mean, if he's so stressed out with work, couldn't he have become a househusband and let her be the breadwinner? It's like that option never even crossed anyone's mind.