After initiation of treatment of B12 deficiency, further damage to the nervous system can be prevented. Whether damage that had occurred already will reverse is less clear, and depends on the type and severity of the damage (if nerve axons have died then they may not recover at all).
Of course, your doctors need to ensure that other processes are not contributing to your pain also - such as secondary arthritis, myopathy (causing muscle aches) or pinched nerves (which can cause your arms to go to sleep particularly when resting on an armchair)
Otherwise, a good rehabilitation program and physical therapy are important to maximise your recovery
Suffering as you are from a severe malabsorption of B12 which has brought about pernicious anaemia you really should be taking your B12 in the most effective form and strength. This means you should be taking B12 in methylcobalamin form at the strength of 5000mcg per day using sublingual lozenges. Your injections which are probably cyanocobalamin? are much too infrequent to have any positive effect on PA.
Whether it is too late to reverse any damage is questionable. But if you really want to stand the best chance of doing so then you must move fast to do as I suggest above. By all means continue the injections if you so wish but alongside the above they are not going to be very if in anyway productive.
I and a lot of sufferers that I know get their B12 methylcobalamin from iHerb.com. I mention this not because I have any connection with the company whatsoever other than as a customer (I live in the UK). They are a good reliable firm but other companies as you probably know are on the internet and the choice is yours. BTW you will also need a good B complex to run alongside the B12 meth so as to maintain a balance of B’s.
I hope for your sake to hear that you have done as I suggest, if not your problems will only get worse.
Take care
Jake34