I think you may have grounds for legal action!!!! How could an x-ray miss three shattered vertebrae? Unless they were cracked in a miniscule way and then collapsed some time after the accident.
I suggest you seek out an orthopaedic consultant and request those x-rays be sent to him to review in time for your appointment. Your gp can and should refer you on the basis of those findings so you can best discuss how to tackle it.
I had a serious accident in my teens and 13 years later, have all kinds of problems. I have also just founded a (slicker looking than this

forum for brits with back problems to meet and discuss, its at british back pain dot co dot uk /forum and is brand new and needs new members who have back problems to share knowledge and support each other. Please join and pass on the word.
Hope you get some answers soon for your back too, must be agony.
Bex