Playing in a minor or major key is an entirely separate thing from playing
accidentals. I think perhaps you are thinking of sharps or flats. A key is a
thing which, once you are in it, remains the same all the way through. Ie, once
you set your sharping levers, you don't touch them again until the end of the
song. Accidentals, on the other hand, are when you play a certain note as a
natural at one point, then as a sharp or flat at another. For this, you have to
flip a lever part way through the song.
I'm sensitive to this because my harp doesn't have sharping levers, and I can't
play accidentals. I can, however, play in a minor key quite easily. For example,
the Major key of C has no flats or sharps. Its relative minor, the key of A
minor, also has no flats or sharps. You can easily play in the key of A minor
without re-tuning or flipping levers, so have no fear of playing in a minor key.
A good exercise to train your ear is to start in the kay of C and play a low C
cookie cutter chord (C, E, G all at once) with your left hand first, then do the
same an octave up with your right hand, while saying aloud "Major One". Repeat
this four times, left, right, left, right, left, right, left, right, then move all
fingers up a string to D, F, A. Do the same thing, saying, "Minor Two". As you
go up one string and repeat four times, you will go to Minor Three, Major Four,
Major Five, Minor Six, Diminished Seven, then Major One. When you get to major
one, you fingers should be on C, E and G an octave up from where you started.
Work and speed and precision, then work the smae thing with arpeggios instead of
cookie-cutters (ie, play the C THEN the E THEN the G rather than all three at
once.) Work at speed and precision with the arpeggios as well. Don't forget to
keep saying the name of the chord aloud as you go. The verbal affirmation of what
chord you are using will become trained into you, so that you can not only
recognize the difference between a major and a minor, but you can more easily
figure out what chords to play where, meaning you will know, in time, whether to
play a Major One, a Major Four, or a Major Five at a pint in the music that calls
for a major chord.
It doesn't happen overnight, but I had a nearly religious experience awhile back
when I was playing chord progression that I had been taugh on piano fifteen years
ago and suddenly said "THAT'S A MAJOR ONE, MAJOR FOUR, MAJOR FIVE CHORD
PROGRESSION!" All of a sudden, it made so much sense!
And this exercise can be done in any key. If you tune your F to an F-sharp, you
are in the key of G and you start with a G-chord, G-B-D. This is your Major One.
Move up a string, you have a Minor Two, and so on. Notice how it sounds the same
as when you were in the key of C. Whatever key you are in, this works. Once your
levers (or lack thereof) are set for a certain key, you don't have to change them
in order to play in that key.
Hope this helps. It did me.
-Laura