Take a big breath, slowly release it back out your nose during the whole thing, and bring your feet back under you so that it is YOU doing most of the standing back up.... not just lying there, expecting him to raise you back up to a fully standing position. Consider him as being there only to guide your body, and support you as you lie back. The standing up part is mostly up to you.
Kind of squat down over your feet on bended knees in preparation for standing back up as you finish up and start back up. The whole time under water is no more than about three seconds, so stay aware of repositioning your feet directly under you right away, and take responsibility for your own weight.
Why not go practice it somewhere in a shallow pool with a friend? After all, standing up in water is something all of us has done a hundred times before... just never from a backwards lying position.