My mother had a stroke in her 30's and taught herself to write using her left hand, though it was always a rather scrawly sort of script. She was a linguist, and always claimed that her command of languages was greatly diminished after her stroke, and, interestingly, that she never really regained a 'mother-tongue' (her first language was Welsh, and she learned French and English at roughly the same point in childhood, and to the same level). This sounds a similar sort of situation to what David is describing. I always felt that what impeded my mother from developing new 'neural pathways' was her lack of patience - she was always a quick and rather impetuous sort of person! So, my very amateurish advice would be, take it slowly and determinedly, but also look for more weird and quirky ways of learning and memorising - this might open up different possibilties. There is a website called 'Memrise' which offers online tuition in various disciplines by means of a sort of multiple-choice / guessing-game process. Might be worth a look:
Anyway, all the best to you, David - and remember, music reaches inaccessible and surprising parts of the brain (and soul!).