In my career as a trainee and fitness professional, I have found that my best education is not coming from books or lectures, but rather day to day experiences. I really enjoy training myself and others, it is an added bonus that I learn more about what I love from doing what I love. The words below are a few conclusions I have come to from the above experiences. Take these as ideas and opinions, not objective fact. Hopefully some of this resonates with you and your training, or provides you with a view not previously explored.
Lesson 1: How My Body Feels is Not a Reflection of How it will Perform
We all have days where the idea of training seems daunting due to fatigue, soreness, pain, etc. I have found on multiple occasions that a session that I expected to go badly due to how I felt actually went very well. As an extension of this thought, I generally find that movement and activity does seem to improve my mental and physical state when fatigued, if only for the time during and right after. Obviously the amount of fatigue and pain is situational, but if the factors are due to the general demands of life it seems that training through is beneficial.
Lesson 1b: It Doesn't Matter How My Body Feels Until the Last Warm-Up Set
Along the lines of fatigue and pain, I have found that things that hurt in between training sessions often do not hurt once I warm up and begin training again. In this case, I am referring to the general bumps and bruises of training, not acute injuries. That being said, worries I've had about pains (sometimes intense) while I'm going about daily activities have been unfounded once I get back under the bar. In my experience, the body can be pretty resilient when asked to perform despite the fatigue and pains accrued from life and training.
Lesson 2: The Performance Effect is Real
I do not usually train with a partner, so most sets are done without a spot or someone watching. In the cases where I do get a spot, or someone to take a video, I find that I have a little extra "gas in the tank" and the lift goes well. I attribute this to an increased level of arousal due to the presence of someone evaluating my performance. How to use this lesson practically? Have someone shoot a video of your PR attempts. The extra set of eyes may help the energy of your lift.
The above is not an extensive list, and I have many more lessons to learn. Maybe you have noticed these things in your training, and if not, perhaps this will come to mind the next time you are experiencing one of the above. Training is not just a process of achieving goals, but also one of self exploration. Be open the lessons that training can teach, and apply them into other facets of life when you can.