Maybe it was also all the nitrogen loading in my system that was getting me so tired.
<snip>
My question is, what do you do to keep from being burnt out?
Doing that many cold water dives in a single day, you may be experiencing symptoms of sub-clinical DCS.
Cold water, strenuous exercise, multiple ascents, all of that can add up. I'd recommend cutting back some on your dives per day and to look into Nitrox if you're not already using it.