I was reading something relevant in a book by Ron Tiner two days ago.
"The images stored in the mind may seem to be visual, but as soon as we try to draw them they become elusive and the resulting drawings can all too easily degenerate into cliches, devoid of any merit and containing little or nothing of the grand designs that the mind originally conceived.
What is needed then, is a means of making accessible the information stored in the mind."
His means are none other than rather redundant "practice, study, and practicality," although he indicates that it is the mindset that dictates the successful transmission from mind to page. Tiner illustrates how you can draw from your memory if you practice enough, as the objects you're trying to draw will embed themselves within your amassed knowledge and experience, and the skill gained will seep into how you draw things other than your choice of study. This should be true to pixel art, and most probably easier with a pixel.
Just keep doing it, accept the initial outcome, learn as you go. (I think the product of trial and error without trying to copy from your mind is most times more interesting than what's planned, but right now that's beside the point.) Eventually you'll be able to translate from your mind to screen satisfactorily. I also think time is important, so spend a lot of it, even when it's "failed". It might just be reshaped into what you want it to be, so don't lose motivation!
If we could initially draw or pixel precisely from our memory, all the savory moments would disappear and we'd be just like printers. Take joy in the natural ineptitude~
