The benefit of your training lies not in the result you get, but in the effort it took to get there.
Improvement through physiological change isn’t driven by how much you lift, but by how difficult it was to make that lift. Not by the time or rounds you got, but by how hard you worked to get there.
Let’s say you squat 160kg and its difficulty was 7/10. A week later you squat 140kg and it was the fight of your life – 10/10 on the difficulty scale. The output was much greater from the 160kg, but the benefit was greater on the 140kg. That’s right, you lifted less weight, but you’ll become stronger because of it. The benefit of the hard fought rep far outweighs the benefit of the easy rep. Struggling through a (perceived) easy task will elicit more benefit than cruising through a (perceived) difficult task. This applies in all areas of life, not just exercise. The concept holds true for both physical and mental accomplishments. The greatest rewards in life often come at the end of struggle.
We need to shift our focus in training from the outcome to the process. Sure, the outcome is important (particularly in competition) and it provides a great measure of achievement (see The Importance of Measurement in Results Driven Performance). But measuring the success of a training session purely its outcome is to undervalue the immense rewards that come from effort.
Bad training days have incredible power over our mental state. Weeks of great training can be derailed by one poor lifting session, or by failure to improve on a repeated workout. The greatest beat-downs to your mental state are almost always outcome based. We are blinded by a lower than expected end result with no credit given to the effort applied, or the training effect of that effort. To combat this, remember that it is effort, not result that determines improvement. Input > Output for Improving Performance.