I think that relaxing the tension could help you a great deal. You could try some mental relaxation exercises as soon as you become aware that you're obsessing over your facial expressions.
Here is the start of my meditation routine: I begin with the top of my head and with each breath, I visualize all the muscles releasing the tension. I imagine it like waves flowing away. Then I move down my whole face, consciously seeing the tension ebb away. I hold a lot of tension in my jaw, so I spend some concentrated time relaxing that area. I find that if I haven't done this in a while, my tension headaches go up dramatically, so it's a good reminder to me to do it regularly.
Now you might want to stop once you get to the neck if you're just concerned with what your face is doing, but it's an excellent exercise to continue down the rest of your body and allow the knotted muscles and tension to drop away.
A lot of it is mental. These obsessions can grab hold of our attention, and that grip can be very difficult to break. With some effort at first, you can use the idea of mind over matter to break away from it.
I know it may sound hokey, but it has really helped me.
Good luck!