Not many takers on this, then. Probably don't want to be the one to say unless you're a 'natural' it just takes practice/experience/time.- You have to build the circuits in the brain to separate the left & right...
Some people find it easier to get the melody 'under the fingers'. Then the bass alone; then put them together. So you could try that.
Then other people say that 'separation and re-integration' just complicates matters and frustrates what you developed as a nice melody by feeling '3 steps back' when then trying to add the bass in.
So just play it all sloooo w l y -
all parts at the same slow tempo, even if you find bits easier... then speed it all up when you've got it cracked... If then making mistakes, slow it down again, etc.
It stands to reason that you can segregate out (an) awkward pieces and apply the method to just that piece, with a bar before and after for integration.
Both methods work... not very well at first. Pick which you find suits you.
Good luck, stay determined, don't get frustrated, pace yourself, you'll be better the next day... etc
