The subfloor and flooring may be partly to blame. Chances are the machine does not sit directly on a pair of joists. This may impart a lot of bounce if the feet are some distance from a flooring support or there is no supporting wall beneath. Closets may have thinner flooring or be lacking the final layer of wood if the contractor was scrimping on materials.
I did a washer/dryer install in our workroom that had minimal flooring. I had to run two 2x4s spanning the machine front and rear so the load was held by the 2x4s and distributed across three floor joists. A couple sheets of plywood nailed to the joists could do the same thing.
My washer is still jouncy right in front of it on spin, but not bad elsewhere. We often have to redistribute the clothes to make sure it's balanced. Washing heavy and light-weight things together or too many heavy items in a load can add to the unbalanced weight flying around. It may be you'll have to run smaller loads and reduce the water level to minimize the weight.
If you have open access to the floor joists beneath and there's no interfering plumbing or wiring to contend with, it might be a relatively simple matter to sister additional joists next to the ones beneath the washer. Even a 2x4 will help. Plywood to span the openings between the joists can be added below to provide flooring support, but it would need support and good contact to do any good. Perhaps just adding a few X braces beneath will help adjacent joists share the load.
Depending on what's beneath this closet, perhaps a floor jack or other means of support might be placed to help support the floor. Just tossing ideas out there that might be doable and affordable.
As to becoming disabled and unable to work, you may be able to apply for social security disability and take early retirement. There are several in my town, mostly they are veterans with injuries, but some were construction workers, etc, who are getting nearly full SS benefits and don't need to worry about it as much.
I'm self-employed myself, so like you I pay the full amount of SS tax on my earnings. If you qualify, you may as well try for it.
-Ed