How can I save my cactus from further damage?
Not sure what is happening so I included a scan describing different cactus problems below. I will send this information from our email portal (firstname.lastname@example.org) since scans cannot be attached here. Hope this is helpful.
General cactus care this fall season: It is very important to differentiate the non-active growth period of your cacti and to give them a rest. From October through February, most desert cacti need reduced water, food and temperature, though direct sunlight should continue.
The NYBG Plant Information Service