Try sitting in front of a mirror with just the mouthpiece. Plug the end with your finger and then try using the side of the mouth to suck air. You may actually have some physical challenge that prevents taking a breath without your lip sliding .. but I doubt it. I think you just haven't used these muscles before in this way and you are having to train your mouth to do two things at once; clamp on the mpc and breathe at the side. Keep watching yourself and keep trying. No guarantees, but you may hit on the right position by trial and error. When you do, you'll see the result. Then use the visual image to reinforce the body memory of the correct embouchure. Hope that helps a bit.