Backache: I;ve tried everything, what helps?

@bergem Berge, it sounds like you have seen a lot of doctors looking for relief from your back pain. Do you have some arthritis in your spine? One question I would ask about your exercises is about core strength. Core strength allows a person to hold their spine erect and stack the vertebrae in a correct alignment that actually decreases the amount of work the muscles have to do for the job. Slouching into a S shaped spinal position will take more effort for some of the muscles, for example your neck that would be holding your head in a forward position instead of balancing it on top of your spine. My physical therapist describes this as a bowling ball on top of a tooth pick. It you balance it correctly, you don't waste effort keeping it from falling off.

From time to time, I get low back pain, and for me it usually indicates that my pelvis is out of alignment. I tend to get pain on the left low back when my left illiac bone (name for the hip bone in the pelvis) has rotated either forward or inward, and it can do that because of joints in the pelvis and a Sacro - Illiac or "SI" joint that allows the movement. The pelvis and lumbar spine are connected with muscles. The answer to this is realigning the pelvis properly with physical therapy and core strengthening to keep it there.

This is a an article for physical therapists that describe complex issues with the pelvis that can mimic a spine problem and even cause sciatic (leg) pain.
https://mskneurology.com/identify-treat-lumbar-plexus-compression-syndrome-lpcs/
This site also has other good articles about back pain.

First I have to say, that everything should be discussed with your doctor in regard to what therapies could help you and be best for you with your present medical condition.

One suggestion I could make about increasing core strength would be equine therapy. If you sit up with good posture on a horse that is walking, your body naturally has to match the movements of the horse and that exercises your back. This therapy helps people who cannot walk by allowing them to experience the same pelvis and back walking movements that they cannot do on their own. If you haven't done this, it would be fatiguing at first, but after a few times, you can get stronger. I experience this myself trail riding with my horses and I progress much faster when I am riding in addition to working with a physical therapist. I am a spine surgery patient for a bad disc in my neck, and having a strong spine supporting all of that from below helps a lot, and reduces chances of causing an adjacent disc problem through wear and tear from bad posture. After a winter of not riding, my back will be tired after the first time, and my strength and good posture just gets better and better as the warmer weather allows me to ride more frequently. You wouldn't need a lot of horse experience to do equine therapeutic riding, because they will have people to help and they likely will choose calm horses for the program. Horse back riding does have risks out on trails, etc, but a therapeutic riding program will have things under control, and you would not just be turned loose on a horse. Often people are walking next to the horse and holding lead ropes, or being spotters on each side if a person has balance problems.

Here is our discussion about equine therapy that you may find interesting.
~ Equine Therapy is Horses helping People, How do Horses Help?
https://connect.mayoclinic.org/discussion/equine-therapy-is-horses-helping-people-how-do-horses-help/
Are you physically able to stand up straight with good posture? Do you have a spine condition that alters your ability to stand up straight? If you look at your standing posture sideways in a mirror, can you get your head, shoulders, and pelvis in line so if you drew a line through them, everything falls within the same line? Have you worked with a physical therapist for your back and pelvis? Would you consider equine therapeutic riding?