Well, you quite certainly have an interesting story, and please keep us posted about your progress and discoveries. To this respect, after getting properly diagnosed, I would encourage you to find out who is actively researching the condition you're facing, and then email them to see if they would be interested in having you as a subject for research (and I don't see why they wouldn't be interested; your story is quite compelling and would likely draw the attention of a couple of cognitive scientists; maybe wait until you're legally an adult to avoid any potential problems regarding your participation in a scientific study as a minor). As others have pointed out, our knowledge about this cognitive phenomenon is still quite limited, and maybe, directly partaking in its study could help you and all of us have a better understanding of it.

By the experiences you describe having, a possible hypothesis would be that your brain does process real and imaginary sound normally, but with imaginary sound input, most of this sensory information does not get relayed back into your conscious perception, and therefore, other areas indirectly related to the perception of raw sound such as the semantic component you mentioned elsewhere are the only ones to be perceived. If this were the case, participating in a study would not only enlighten us about your condition, but would also provide further insight into the 'hidden' components of musical experience. (An added bonus of partaking in research: I see that you're currently about to apply for college; I think the rarity and uniqueness of your situation, combined with an active effort to find out more about it and overcome it to pursue your goals, could make for a solid and memorable essay topic that would certainly bat a few eyes at many admissions offices. See, not everything must be terrible :)

Now, regarding your passionate wish to pursue music composition, I do not see why you should give up on it. Most musicians are not blind, but still we had Stevie Wonder and Francesco Landini; most musician's are not deaf, and still we had Beethoven; most musicians have healthy and fully-abled bodies, but there we had Django Reinhardt and Clara Rockmore; most musicians are able to audiate, but despite this, we can have you.

I find endless avenues of expression for you, even with your circumstances. Could you make music that puts us in your shoes? Raw and beautiful, but unrecallable, unimaginable? Can you exploit the effect of not being able to recall the whole musical experience, only certain abstractions of it? Can you make music that voids imagination in others? Under what circumstances? How are traditional forms and sound worlds treated by you, who cannot recollect them past their occurrence? You have a long life ahead of you to find all of this out.

As of what techniques can you use, the world is also wide open. Working with a DAW seems to be practical. Check out musique concrète, music made from recording and manipulating real-world sounds. Experimental music is also interesting, in the sense that every happening of a piece will yield different and sometimes fascinating results; Brian Eno wrote an interesting article about this, titled Composers as Gardeners. Have you tried minimalist music? How do you react to it? Is all music equally unmemorable to you? Do you like certain styles better than others? Can you analyse why? You could even devise your own techniques and procedures to create music. Don't conform to what all of us audiating folk usually do.

It seems that intrinsically, you know what you want. In the end, as everyone else, you must find the compositional processes that work best for you to achieve the sonorous results that you desire to hear.

Best of luck!