I’m sorry you lost your mum (and dad, or have I misunderstood?), it is very hard when a beloved parent dies. And then to also lose your much-wanted baby is an awful lot to deal with.
It sounds like your dh was heartbroken too.
It must have been hard for him to see you suffering, and I can understand why he’d be worried about having another baby if you have been struggling with anxiety and fighting a lot. It doesn’t seem like a nice stable environment to bring a new baby into.
what does “run its course” mean to you? Do you still love your dh? Does he love you?
If you and dh kept trying for a baby but were unable to have more kids, what do you expect your feelings would be?
I am a pragmatic person and I would say you need to get yourself some counselling and find some perspective. If you split up, at the age of 37, you have to face up to the reality that you may not find a new partner in time to have a fourth child. Also you’d probably lose contact with your 3 boys for up to 50% of the time. You’d have to leave your big house, most likely.
is separation really what you want after 18 years of marriage?
Think long and hard before you make those threats, just in case you change your mind but dh is inclined to agree.