Not being able to go backwards makes for frustrating moments; if you go ahead of your arm by accident, you're done for, there's nothing you can do to stop the bleeding. Additionally, if you get yourself right next to a wall, there's also nothing you can do to progress: You can't jump without shooting your arm, and then your arm blocks you from jumping.
All in all, the controls are frustrating. Perhaps it could be improved by separating the moving and shooting controls. I know you were going for Only One button, but you also have Only One projectile.