I'm going to stake my admittedly-nonexistent internet reputation on saying that using willpower alone to suffocate yourself is not going to work. Not in the bathtub, not with a pillow, not with a plastic bag. The entire premise of the inert gas method is that you need a workaround for the instinctual panic that sets in when your blood CO2 level starts to rise. You do occasionally hear about bathtub deaths among able-bodied adults, but they all seem to break down into three categories: natural causes, drug or alcohol blackout, and murder.
I am impressed with the determination of some folks in this thread, though. I don't know what causes a whole-body charlie horse, but it sounds like you'd need to expend a lot of effort to get one.