I used to do the same as well. In my case it was a result of my swimming background. Swimmers always keep enough air in their lungs (helps them float better, but more importantly use it as back up if water splashes in their mouth during an inhale to be able to blow it out). Hence in diving before a drill I would take a deep breath for backup too and as a result I would ascent as a rocket. With practice underwater things improve a lot. To get used to it, you need to do whatever you do very slow. So slow that one breath is not enough, hence you need to breath in and out at least few times during the drill. You slowly get used to the idea that air is available at any moment and hence you don't need to keep your lungs full. Relax, take several normal breaths and do the drill slowly one step at a time. Soon it becomes so natural that you won't even think about itI am a newb, but I tend to unconsciously hold my breath when doing tasks, especially mask removal. Take mask off with left hand, remove and replace regulator with right hand, put mask back on, and I am 4 feet higher than when I started and still rising...