I can relate to your troubles but unfortunately there isn't anything you can do now. In 2012 I applied for DLA and was awarded the lower rate of care and mobility, but was turned down for the higher rate of mobility because my condition had been diagnosed as psychosomatic, so the doctors basically thought it was "in my head".
Fast forward a year later and intensive investigations resulted in a new physical diagnosis, and I had no less than 5 hospital letters saying so and a very supportive letter from my GP confirming that at the time I claimed in 2012, I had a physical condition that was never psychosomatic. But I couldn't force a supersession or revision.
In the end I had to bite the bullet and apply for PIP using my new diagnosis and was awarded the enhanced rate of care and mobility going forward, but no back pay.
I am luckier than you in the sense that next month I will present my case to an independent tribunal because I reported this to DWP before the end of the 13 month window, whereas you're out of time to do so, but I still couldn't get DWP to budge and revise or supersede their original DLA decision.
