You need investigations - your progressively worsening and your problems are becoming more and more systemic not muscular.
If they are worse with flexion then you could try extensions - it's a mckenzie approach HOWEVER with your history I would would be getting investigations to ensure that you couldn't hurt yourself attempting the extension protocol and I don't hold out much hope that it would make a difference... it's more of a if it isn't going to harm you then give it a whirl.
Go with physiobob and get a good investigation - if your doctors and GP's aren't working for you, then shop around and find one that will advocate for your pain. Ensure they know how it started and how it has progressed - any systemic symptoms like weight loss and unwell feelings or any neurological symptoms like numbness or clumsiness even loss of bladder/bowel function make sure you mention to the doctor.

good luck for now, find a doctor who will advocate for your health.