Q. Unstable jade plant
This sounds as if it's a very large plant if it's in a 21 inch pot, and it is unstable and fell over. It's probably beautiful, too. I know how you feel about a plant that you have had for so long, and you don't wish to continue to cut it back. You also say you think it's running out of growth room. The most logical answer, then, is to repot it, maybe into a slightly wider and deeper pot, using enough soil to set it into to provide the stability it needs. Good luck.