Go and ride a few fifteen meter circles or a turn into the short side of a ménage. If you want to learn diagonals then trot the circle on the right diagonal and then once on the wrong diagonal. Keep doing that. Don't make it a habit to look down. It will take forever to actually learn feel. You will start feeling the "wrongness" of being on the incorrect diagonal. As for sitting trot-that's only so that when you start trotting you get the feel of the correct diagonal. It helps but after a while you will automatically just start on the right diagonal without knowing.