When you understand that you are asking brain/body to grow new brain cells and pathways perhaps you can be more understanding/forgiving and patient with the process. Sometimes these things take a lot more practice and time for those new neural pathways to be made. Sometimes it just doesn't happen.
Practice with doing scales is a predictable exercise; however, performing a piece with all those less than predictable changes is quite hard.
For example, here's a classic 2-hand independence exercise: using your left hand, rub your stomach and with your right, pat your head simultaneously. Now switch hands.
This activity/exercise can't be done successfully by 95-98% of us. With practice, you can better at it...but most of us can't master that exercise. This piano-playing you are attempting is analogous to that.