Hi Roxie, this is a rotten thing to happen as a lot of us seem to put to much trust in the medical field but you can turn this right around in time in your favour.
I don't know much about injectable vitamins and minerals for nerve health but i do have shots of b12 [ Hydroxocobalamin ] that was ordered from my neuro and i continue to have them and what a differance it has made.
The cause of my PN was low B12 & Prediabetes, i had to fully change my eating and drinking habits and exercise, i couldn't walk for to long on a pavement because of my burning feet would kill me, so i went to the local swimming pool and just walked up and down the unused lane of the pool [non weight bearing ] i used to get there when they opened at 6.am, best time because one, its wasn't full of normal swimmers, two there were surprisingly quite a few others doing the same for all sorts of med problems and it helps talking to others as well.
I changed my diet to a healthy high fibre diet, stayed away from sugary carbs and stopped drinking alcohol [beer] as its full of sugar and the alcohol is a well known nerve irritant as well.
Now, i am fast fowarding this to these days, 4 years later.
No burning 98% of the time, no pain, prediabetes is gone, complete numb spots on my feet have regained feeling, i can wear leather shoes and boots no problem, everywere my neuro tests he gets a positive reactions, i feel alive again all day long and feel full of life, a complete opposite to what i was originally, when i struggled to make it to lunch time, so so tired all the time.
So if i can do this so can you, keep in mind whilst healing, flaring of symptoms is common but our bodies have got a remarkable ability to repair itself.
good luck
Brian