I've been riding mountain bikes for 12 years and while I am certainly not the fastest guy out there, I like to think my technical skills are pretty developed.
That being said, there are some mornings where I just don't "feel it", you know?
Taking forever to get warmed up, or tired, or maybe drank a little too much alcohol the night before, etc. And if I ain't "feeling it", then I'll skip the difficult stuff, and get my miles on another trail. Of course, there were also times when I felt like I could ride anything all day long, and pitched myself right over the bars on some innocuous little obstacle.
I just don't see any point in pressing my luck. Listen to the little voice(s) inside your head.....lol.