I think maybe this is the key sentence in your post:

If it was a neurological condition like epilepsy or Tourette Syndrome, it's unlikely you could confine your jerking, convulsing and hitting yourself to those times when you were alone. If you do this voluntarily to ease your anxiety, it could all be a complex form of self-harm, perhaps related to OCD behaviors, to reduce your anxious feelings. It could even be faux seizures, which are psychologically induced rather than neurological in nature. There are many possibilities. But the fact that you can confine this behavior to the times you're alone does make it look like it might be an emotional or psychological problem that you can get help with.

Do you have a treatment team of any kind? If you do, it might be important for it to include both a psychiatrist, who is a medical doctor, and a therapist.

If you don't have a treatment team, you probably need to find a doctor, either a primary care provider or a clinic of some kind. A doctor can run basic tests to make sure you're generally healthy. It would be good to ask for a referral to a psychiatrist and therapist. You might need to see a neurologist, too, just to check out whether this is neurological (coming from your brain and central nervous system) or psychological (coming from your emotions and thinking) in origin.

I'm sorry you're going through this. It sounds very difficult. But the fact that you do it voluntarily when you're alone to reduce anxiety does make it sound like it's something that could be helped by therapy. It's possible that you start it voluntarily and then it takes on a life of its own, running its course until you run out of steam.

If you don't have medical insurance, it may not be too late to sign up under the Affordable Care Act (ObamaCare.) If your income is limited you may be able to get free or very low cost medical insurance that will cover many of your expenses, including giving you access to therapy. I wish you the best.