i've actually been doing that AP method you were talking about, but the issue is that it seems kind of one-directional. as in, i can think 'D' and hear it in my head, but if i hear a D, i won't automatically make that association backwards and identify it as a D. any pro tips for this
you must learn to conjure the memory even while other notes and keys are playing. if you use your voice also that can help guide you part way there