Why is this cactus drooping? I tied the top so it would stay upright and not droop, but now it’s starting to droop anyway over a period of two months. The plant belonged to my deceased fiancé and I don’t want to lose it!
Looks like a fairy castle cactus, which is scientifically called Acanthocereus tetragonus. Like @elPolloLoco already commented, the drooping is probably caused by wet roots. This cactus needs a lot of bright light, and very well drained soil. Only water it every time when the soil completely dries up first. Here is more info about the care of this plant.
I think @B540Glenn was close. I think this is actually etiolation caused by insufficient light/water/food at some point in the past. Note that several of the stems have a very narrow section about 4" below the top (right where the bend is). The newer heavier growth can't be supported by this thin stem.
Not sure what your preferred remedy is. It might be better to take a healthy piece(s) and root them and start over. Or you could trim all the stems with narrow parts and the plant will grow new stems and cover up the snipped parts. Or you could leave it alone.