I've had this cactus for a few years, and for a while it's had a problem where it grows so tall that it can't stand up. You can see in the photo that I have it tied to some stakes to keep it upright - if not for those it would literally uproot itself in under a day.
Why might this be? I don't know all that much about houseplants. I water this cactus (and my other cactuses, which are not the same species) once every two weeks, about 1 to 1.5 cups of water. I use some cactus fertilizer like twice a year, pretty inconsistently. It lives perpetually indoors with those three light rods visible in the picture as its sole light source (On for 12 hours a day).
Given how little I know about proper plant care, I'm sure none of that is ideal - but is any of it the obvious culprit for why this happens? What should I be doing better?
Thanks for any help.
P.S. Those two nodules just above the lower string are brand new, and it's never branched out like that before - what should I expect them to become? Round bulbs? Branches? Flowers? Nothing at all? The tip top of the cactus being white is also very recent.
I'd say I'm even more sure about my theory. It's definitely a mammillaria. Monkey tail have long, almost fuzzy spikes and are thinner, hence the name. Rat tail cactus look more similar to yours, but again, they are very long and skinny, like, well, a rat tail lol.