A Question For The PTs

I got a little frustrated in Crossfit last night.  Last night was a strength workout, working on shoulders.  The WOD consisted of 5 x 5 push press and then 5 x 5 push jerk and as it was a strength workout you had to lift as heavy as you could.  Theoretically, you should be able to lift a smidge more for the push jerk as you are using momentum to help and that was the case with most people in my group but I actually went backwards.  My problem is that I fatigue easily, this happens pretty much in every Crossfit class I do regardless if it's strength or cardio - I'm good for a small period of time but I crash and burn easily.  I've tried playing around with food but have gotten nauseous a few times.  So, does anyone have any tips to increase stamina?