I also found the frequency of the tone made a lot of difference, probably because my hearing is not the greatest.
My bet is that would make you "normal", then, as far as learning morse is concerned.
I have previously suggested, as a hearing test, listening to the morse then just repeating it back to yourself - WHILST listening for the next char at the same time, but not trying to decode the first char.
If you can't hear it to repeat back then you won't be able to decode it.
In my view, the repeating back means that you are keeping it in your memory, and you may well find that after a period that keeping it in your head means bit you are decoding quite a bit of it without even trying - which is probably what you are trying to achieve . . . the char just pops into your head seemingly from no-where.
Also, this mini two-things-at-once exercise in keeping the morse code in your head, may well be a first step towards head decoding; certainly it is a component.
The main issue though - is to make sure that your ears are getting the raw material.
We can all remember tunes etc - so I don't think the learning is the main issue,
and OC is certainly putting in the time, and has been doing so for a year or more;
so I think we need to work through the whole process from end to end, just to be sure . . .