Here we have a 2 part question.
Q: I have a cactus tha is over 15 years old. I was watering it the other day and was picking it up when it almost broke in half. What should I do to keep it alive.
Can you email me a photo? If it is a big break it may be that you need to cut it off and re-root it, if it is a species that will re-root. If it isn’t you will need to “splint” it and hope that it will heal up strong enough to support it’s weight. But a photo would help me give you better advice.
…and, yes, here comes a new email with photo… so now we get the rest of it….
I would recommend repotting in fresh cactus soil, mulching with a small rough gravel like lava or small crushed rock and laying the whole cacus on it’s side on top of the gravel, it should root along the length and then grow new “pups” along the length and turn in to a many headed cactus. Do not use smooth gravel like aquarium gravel as it stays too wet. It does look like it would like more light… so try moving it closer to the window or to a south or west facing window.