Difficulty Playing Standing Up

A bit frustrated at the mo, just learning the Cowboy's from Hell (Pantera) solo and can do the long stretch descending pattern just fine, however, when I try play that standing up, my wrist is at such an awkward angle that I cannot play it comfortably at all.

This doesn't just apply to this solo/song, but to just playing when standing up. My guitar isn't at my feet or anything, but above my waist, and it still causes my wrist to go to near snapping point when I play standing up.

Any suggestions on how to solve this?

Cheers.