I think it goes without saying that sports teaches us a lot about life. These are some of the things I think about -- parallels between mountain biking and life. Hopefully they will be helpful to someone.
Focus on the trail ahead of you. Don't look behind you. Take a look at what is coming up a ways down, but don't forget that it's what's right in front of you that may/will hurt/kill you.
The hill may be steep and you may get very winded. Take breaks and keep going. Your lowest gear will get you there. What looks like the crest may be the top. If it is, take a look back (the one time you can look back) and appreciate how far you came. If it isn't, keep climbing. Enjoy the ride down.
Fear is a hell of a thing. Being cautious is a virtue, but letting fear control your decision making holds you back. Some of my best rides have come after I was down and out. I didn't care what happened in the moment. I put all of my mind, body and soul into riding the trail. And with it, I was able to ride features I had been fearing with no problem, or completing a route much faster than usual. Of course, if I got hurt, I would have something more worth worrying about!
Rider 1: "Great ride!"
Rider 2: "Meh, I fell a few times."
Rider 1: "Did your bike get damaged?" (the most important thing)
Rider 2: "No"
Rider 1: "Did you get hurt?"
Rider 2: "No"
Rider 1: "Then it was a good ride."
Sometimes you just need to let go (of the brakes).
Mountain biking is a form of meditation. You have to keep a clear or positive mind. Most of my crashes came from a negative thought entering my headspace.
Mountain biking is more mental than physical. The inability to get out of your own thoughts will cause hesitation and too much caution and not enough risk taking. This can make the ride boring and prevent you from progressing.
Don't get complacent. Most of my major crashes were not on advanced trails, but beginner trails that I had ridden hundreds of times.
Don't follow other peoples' brake skids ("footprints"). You will get stuck and go out of control off the trail. That creates pain. If you must, create your own mistakes/pain. You'll at least learn something.
You can spend thousands on a bike with an amazing suspension system, but there is no better one than the human body, so take care of it.
Don't be intimidated by the loud riders with the fancy bikes, wearing fancy gear (bonus points if they're all wearing the same shirt). They're usually the worst riders. They will stop on every switchback to celebrate each momentous victory. You'll end up passing them and leaving them in the dust.