Q.Dying Cactus
I had a cactus that fell over and the top split open and was not healing properly. I cut the top portion off in the hopes that it would heal, but after it dried out some, the cactus has started rotting from the top down again. What should I do?
I would cut back any watering you may be doing. Without seeing the injury I can only suggest you make a clean cut and allow the cactus to callous over.
You can dust the cut with cinnamon, it acts as a natural fungicide.
https://www.gardeningknowhow.com/garden-how-to/info/using-cinnamon-on-plants.htm
It may be that the damage is to great to over come but many Cactus can be propagated from cuttings.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/removing-cactus-pups.htm