How to get out of the habit of simply moving your head back to get away from a punch?

Realistically.. you wont be able to teach it to yourself so much. Work on the motions used when slipping punches, but it will take a fair bit of sparring for you to get comfortable to slip punches in practice. But getting your body familiar with the motion you want it to make when slipping helps to make the process a bit faster and stops bad habits.