I think that all marriages go through difficult times. After a baby is born, work problems, family illness , elderly relatives. That’s life. A mature person know this and tried to support their partner and make it work.
A decent and honourable person talks to their partner and tries to resolve the issues ( get help, go for counselling , change their own reactions etc ). Like @Curiousmum69
A bad and selfish person decides avoid the issue and shag someone else.
Most of us got married and promised that would do the first and not the second. If he had wanted the kind of causal thing where you just walk away, HE SHOULD HAVE TOLD YOU THAT UPFRONT.
But he didn’t, did he? He Wanted you to be committed to making it work and Being in for the long term while he got to bail out as soon as he thought he had a better option.
OP you DIDNT cause your marriage to end because you had health problems after you gave birth to HIS CHILD. That’s just his excuse. If it wasn’t that it would have been “ work stress” or “mid life crisis” or his gran dying or hearing that his best friend from primary school had died in a car crash.
OP you really REALLY need to read the chump lady website.