Know thyself: understanding your own limitations
One of the most useful habits I've ever acquired is frequent and brutally honest self-assessment. Getting to know myself, learning more about my own personality, and understanding my fears and anxieties sounds like pretentious psychobabble and more than a little narcissistic. But it has helped me grow in ways that would have otherwise been impossible, because it helped me find my own limits--and push past them.
In defensive training, my personal limits have sometimes been caused by underdeveloped shooting fundamentals. Sometimes they've been physical, either because of adverse conditions or a lack of adequate fitness. Sometimes my limits have been entirely self-created; whether I convinced myself I would fail a shooting qual and created a self-fulfilling prophecy, or told myself I wasn't good enough to teach a certain class, the problems I struggled with were entirely the product of my own mental and emotional limits.
The cool thing about figuring out what makes you tick is it allows you to change things that don't work well and rebuild patterns of behavior and thinking that work better. For instance, I know that I get anxiety before any shooting qualification, no matter how easy it should be or how many times I've shot it before. Understanding this about myself has made me much better at passing shooting quals--I anticipate the anxiety beforehand, deal with it via relaxation exercises as it's happening, give myself time for the adrenaline to wear off (and my hands to stop shaking), and then shoot it when I've calmed down a bit.
It's also extremely important to know your limits so that you aren't tempted to do things that you are completely unqualified and incompetent to do. If you aren't an expert at hand-to-hand martial arts, you shouldn't count on your ability to win a fight with your fists in lieu of carrying a gun. If you carry a gun but have never trained with it, you shouldn't expect to draw and shoot it perfectly when you need it most. If your shooting knowledge doesn't go much beyond the very basics of pistol shooting, you shouldn't offer to teach your friends high speed combat tactics in your spare time.
When we know ourselves, we know our own deficits of knowledge and can then act accordingly to improve. We know where we are lacking in certain skill sets and can get more training. We know what we don't know and can read more books on the subjects we want to know more about. We can train harder, and more importantly, smarter, and we can make every single training session count. When we understand where our fundamentals are lacking, we can correct them with specific drills. Instead of spending hours wasting money on throwing lead downrange and making lots of expensive noise, we can ensure that every round counts--every effort was intelligently directed toward a precise goal. Precise goals and focused methods lead to consistently better results.
Get to know yourself and make a habit of reassessing your own personality and character on a regular basis. By treating yourself as both your own student and evaluator, you will have a much better grasp of what you already do well and what you desperately need to work on and improve.
Be kind to yourself, be honest with yourself, and stay safe out there!