My best training sessions are often on days when I feel less than motivated at the start.
Also, when I go into a training session relaxed and with the attitude of low expectations, I have a killer workout, as opposed to having the attitude of conquering the world.
Last night I was considering bagging my session because I was tired. But I figured that I would do it anyway, but not sweat it if I didn't have a good workout. I figured doing something was better than doing nothing.
Turns out that I had one of my best sessions ever.
Projecting on how my workout is going to go based on how I feel before doing it doesn't seem to be a good indicator for me. I'm better off just doing it. If I'm hitting a wall, and everything feels heavy, I'll bag it and live to fight another day. But more often than not, I end up having a good session.
There's a fine line between working through resistance to get the benifits of working out and pushing yourself too hard, and experience will define where that line is.