I tend to kill plants, unintentionally, and so in April I bought a kill proof plant – a pincushion cactus. Today, I’ve just noticed the cactus is dying. A brown withering of the nodules is spreading from the base of the plant up. I keep the plant on my windowsill to get light and I don’t water it very often. The weather here has been very unpredictable – hot and sunny one day, rainy and overcast the next, but it’s quite humid here and the plant survived winter fine. What am I doing wrong? I love plants but if I kill even cacti I’ll need to give up in disgrace.
No such thing as a kill-proof plant (sanseveria, the snake plant, comes the closest, I think,) and cacti are very easy to kill. Most people either water them too much, or not enough. Here's the thing you're not doing - you're not checking the soil moisture in the bottom of the pot before you water. This article will give you some ideas about that: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
Kept indoors, it will need to have the soil get completely dry all the way down. However, as soon as it gets dry, it needs a thorough watering, meaning till water runs out the drainage holes.
Try again. Spreading withering doesn't sound good, but please don't be defeated. Keep trying, and if you learn to test the soil before watering, you might find a whole new world opening.