Sustained Grip Help

My Forearms seem to fail first during all of my workouts. I dont think they are weak, I just cant do alot with them. During my last workout 100 of dips and pullups. I was able to do 15 pulls and then after that could not do any more then 5 per set bc i couldnt hold on the the bar.

Any suggestions for correcting this? I have never done any specific arm training so I have no idea…