Tunnocks, I have been where you are. DH died when the DC were 8 and nearly 4. The hardest time was when DS was 11 or 12, and he was horrible to his little sister all the time, and she would wind him up, and I felt like I was constantly in the middle trying to stop them bickering and fighting each other and fighting me for my attention. They both adamantly refused any kind of counselling.
We went for months when I could not have them at the dinner table together - I would send one to go and eat in front of the TV instead. I sometimes had to do emergency stops in the car to shock them into silence because they were bickering so much in the back seat it was distracting me from driving (don't worry, I always checked the rear view mirror first to make sure it was safe).
I didn't have much support (moved countries after DH died, and my parents were already elderly and progressively more disabled by then). It was a horrible time.
But - it got better. The DCs grew up a little and stopped bickering - they are good friends now. I managed to find some things to do just for myself, rather than only running around for other people, which was healthier for me and for them.
When you are in the fog of it, it seems endless, but there really are better times ahead. Obviously there are issues now that there weren't for me - social distancing, bubbles etc - but I did learn to ask for help when I really needed it, and I think you need to do that too, whether it is from friends, family, GP, other support services or whatever.