Am I gripping the handlebars wrong?

I completed two runs at Downieville, CA yesterday. It was extremely challenging, about 45 miles of rocky downhill.

About 1/3 way through the first run my fingers and hands were absolutely killing me.

I usually ride every weekend and never have this much pain.

I was gripping somewhat tight but only because it was so incredibly rocky I didn’t feel like I could loosen my grip.

I usually do a full grip on the bars with two fingers on each brake.

I’m somewhat new to the sport, only riding since March and new to a FS system.

Was riding a rented medium specialized enduro 29er. I’m about 5’11” and weigh about 185lb.

Any tips? What am I doing wrong? Everything felt pretty good except for my damn fingers, hands and wrists.

Thanks for any tips and advice MTBers!