As the others have said - Bouyancy is the issue. All those instinctive things you do now don't work!
Try to have the minimum gas in the counterlungs, makes things easier, but its easier said than done!
As said above, it does seem like a big mistake, but stick with it. I originally did my coarse without owning a unit, by day 3 a KNEW I HAD to HAVE ONE.
Gareth
Rebreather novice & numpty
