Yeah man I feel you.
I used to have similar (may a little less intense tho) fatigue issues after full days of riding.
I used to get up at 5ish am, drive up to Bear, get on the lift by 8:45ish, ride until 1:30 pm, get lunch, then continue riding from 2:00 pm to 4:30 pm. Then drive back home.
Days like that definitely left me feeling pretty beat
--
Now I like to take more breaks and ride a little more laid back, just having a good time, not trying to squeeze every last ounce of snowboarding out of the day like I used to...
---
By the sounds of it though, I think you could probably benefit from exercising regularly (like run 2 miles every other day, to increase endurance) and eating healthy. Big breakfast high in protein and then follow it up with 4 small-sized meals spaced out throughout the day. Should keep your body fueled pretty well and at least take care of those headaches
Also, sometimes, I like to post up in my car after lunch (round 1:30 pm) and take a 30 minute nap...pretty refreshing...