Kat I am deeply sorry for your loss and your family stresses and pain. We lost my brother to a virulent form of kidney cancer five years ago -- 90 days from diagnosis to death, so it was a shocking end but not as shocking as your own loss -- we had some time to prepare. Based on our experience, I'd say that you are still in the early stages of grief -- it took our mother almost two years before she could function at all and four years before she could barely even speak of him without collapsing in grief -- he was her "baby" and they were very close. Now at five years she can occasionally speak of him briefly without instantly falling apart.
I don't have to tell you that you've suffered two terrible blows: the loss of your son and the awful injuries of your daughter, plus the confounding complication of your ex's behavior.
No one can say exactly how long it will be before you see marked improvement in your spirits. Grief takes time and it takes a toll. There's no way around it. Healing is different in every person, for every loss. There is no "right" speed and there is no "right" way.
You're correct that he'd want you to take excellent care of yourself -- he loved you, right? My brother asked me dozens of times while he was dying, "Are you going to be OK?" Our hearts were breaking for him, for his terrible suffering, and he was worrying about all of us! I told him, "Yes, I'm going to be OK. It's very, very hard, but I'm going to be OK." I'm sure your son would have asked the same of you, if he could: "Mama, are you going to be OK?" He'd want the answer to be "Yes." Eventually.
My mother found a grief counselor and a grief support group at the local hospital very helpful after a few years. Is there something like that available where you live?