set your metronome quite slow, and only pick on the beat using all down picks. make it so that its way slower than you can play.
Then using just one string and fretting just one note do a few downs on the beat and notice that each time you down pick you are having to come back up for the next down pick, now just make sure you hit the string on the up pick. Once you have that rhythm going, on that one note one string, keep it flowing and start the exercise again.
Think of your picking as beign like tiny strums .. keeping it moving and maybe even exaggerating how far your pick moves to start with so that you start to feel the constant rhythm and pace, maybe even say "down" each time you hear the metronome click too.
its tricky and you have to go a lot slower than you might normally, which is frustrating, but you quickly become used to it and will be able to speed up.