Not sure why it's been limping, the baby one also started limping and died recently. The bottom is a bit yellow but not rotten yet. Not sure how to care for it not that it's so tall
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Your photo is very small. The plant looks like it needs more light as a first suggestion. Please look for some cactus care information online. there is much out there. – user22542 Feb 17 '19 at 13:15
Cactus generally require lots of sunlight everyday. The only way to "fix" this plant is to put it in a place where it can get a minimum of 6 hours of sunlight. It will never look normal.