Never regret. Always reflect.
This is one of those mottoes that applies to training, and to life. Regret and self-flagellation don't make a better you - they just make a more broken you. Reflection, on the other hand, in both good times and bad, make a you that's better prepared for future action and reaction.
One of the greatest tools in your toolbox should be your training log. Without a log, any potential reflection on your training is going to be severely limited. Keep a log handy and record sets, reps, weights, exertion, and other notable stressors and restoratives. You don't have to live in it while you're at the gym (I see my share of people who seem to spend more time staring at their notebooks than actually lifting weights), but spend 10 minutes or so to record the sessions numbers and add a few qualitative notes. The reflections will help you make sense of subtle dips and crests that emerge over time. It will also help you make sense of those times when (heaven forbid) you find yourself in plateau-land or on the DL - helping us look at the bigger picture when it's especially hard to step away from the "WHY ME?" mind-set.