Hello Brassinpocket,
Welcome to this forum and the wonderful people who post here and try to help others.
It HURTS when I read of those in their late teens and twenties who like yourself are having chronic back pain.
Brassinpocket, what I was hoping to read in your post but did not see was that you had been to a neurologist or spinal surgeon. GPs, etc. are okay, however, in my opinion you really need to seek the advice of a neurologist/spinal surgeon.
Look for the best spine specialist (with MD after their name) that you can find. Talk to those you know and find out which spinal specialist they had high regard for and which ones they did not. Call your local hospital and ask the staff there which neurologist/spinal surgeon they would go to if they had chronic back pain. If you live in an area where there is a professional sports team, call their office and find out which spinal surgeon they use for their players.
To avoid any misunderstanding, I recommend that you see a neurologist/spinal surgeon, not because you need to have surgery, but rather because THEY are the most knowledgeable about backs. You need their expertise to determine what the problem is AND then for the doctors to determine what course should be taken.
Once you have your list of highly regarded spinal doctors call the one at the top of your list and make an appointment. Then call the one that is No. 2 on your list and make an appointment with that doctor. It is always a good idea to have a second opinion.
May I suggest that you not practice your Pilates for a couple of weeks and see if that helps.
In the meantime, get your list of neurologists/spinal surgeons together and make those appointments.
I wish you the best. Do let us know how things go for you.
Take care.
RichT