I'm sure you have thought it through or about it but I was actually being serious that third time. I'm not talking about becoming Maria Callas or Nat King Cole, just enough to diffrentiate the notes. I can think of no scientific, moral, political, spiritual or sarcastic reason why you could not learn to diffrentiate 13 notes. With your voice.
I used to notate for my friend that played flute. We'd go through the part and I'd write the letter of each note. It was labourious but she was both patient and quick and eventually, we'd get a great piece down.
Another thing that I noticed when I hit 48, I'd hum parts a semitone flatter than they were meant to be. So my sax playing friend would be looking at me all funny as if to say "this is awful discordant shit" but in desperation, I'd say, OK, shift everything up a semitone" and suddenly you'd be getting the sweetest sounding sequence of notes. Weird. So I watch out for things like that now.
I can understand your frustration but I believe that itn is by no means an insurmountable problem. The solution will just be labourious.......but worth it.