It may be too chilly. I just looked at the website where you purchased your plant. Four things stand out: keep it warm, water sparingly, keep it in bright light, use cactus appropriate soil. I do not agree with their suggestion to put a layer of gravel in the bottom of your pot.
https://www.bakker.com/en-gb/p...
Have you looked at the white spots with a magnifying glass?
I visualize you never having children because you failed with one difficult plant.
