What is the hardest swim set you have ever done?

Oh man, it hurts to reading that!  I know exactly what you went through, because that happened to me on a 100 breaststroke race.  I hadn't yet been diagnosed with dysautonomia, so I didn't understand what was happening.  Adrenaline had over-flooded my system, and I lost muscle control like you did.  My body locked up on me about 10 yards before finishing.  Those were the hardest last 10 yards I have ever swum.

Exactly  
Good you have the dysauto under control now

That last 15m lasted about 30 seconds and the only thing that kept me from drowning was the split request