After a few years, if you practice quite a bit, you get to where if you can hear it in your head, you can more or less automatically translate that to finger movements. A seasoned player doesn't really have to think much about the mechanics. You think a melody, and then it comes out.
That doesn't mean that composing becomes automatic -- often once you cross that chasm, there's still frustration between having an abstract musical idea, and translating it into actual notes. Also, by that time, the way you listen to music has changed pretty drastically, and you also realize that a lot of your own ideas are mundane in comparison to the things you can half-way understand in music that you listen to. That's where most of the work actually happens -- in becoming better at thinking about musical ideas.
If you're able to sing the ideas that you've got, I'd recommend recording yourself singing them, and then spending the time learning to play those ideas. That at least lets you work on the gap that you mentioned between thought and mechanics in a concrete way.
For most instrumentalists though, myself included, you pretty quickly get to where you can play things more easily than you can sing them. If you ask a trumpeter to sight-sing a piece of music, you'll usually see them miming the hand movements because they can imagine how it will sound when they play something, and work backwards from there to singing.
That doesn't mean that composing becomes automatic -- often once you cross that chasm, there's still frustration between having an abstract musical idea, and translating it into actual notes. Also, by that time, the way you listen to music has changed pretty drastically, and you also realize that a lot of your own ideas are mundane in comparison to the things you can half-way understand in music that you listen to. That's where most of the work actually happens -- in becoming better at thinking about musical ideas.
If you're able to sing the ideas that you've got, I'd recommend recording yourself singing them, and then spending the time learning to play those ideas. That at least lets you work on the gap that you mentioned between thought and mechanics in a concrete way.
For most instrumentalists though, myself included, you pretty quickly get to where you can play things more easily than you can sing them. If you ask a trumpeter to sight-sing a piece of music, you'll usually see them miming the hand movements because they can imagine how it will sound when they play something, and work backwards from there to singing.