Bad Days at the Gym

Both the stress of training and the stress of feeling bad about a training session cause your body to release cortisol and other hormones that make it more difficult to recover for your NEXT workout. If you just try to push through, you get caught in a loop of missed lifts and bad feelings.
The secret is to (A) realize that, and (B) find a way out of the loop, either by backing off on your training or addressing the negative feelings directly, through meditation, by having fun outside of the gym, or however else you deal with stress in your life.