I'm sure all of our parents have told us, "Just do your best," at one point or another, and for a while, we believed that was good enough. Especially when we got participation awards and gold stars for doing what was expected of us anyway. But now that we're older, at least I hope you've realized by now; sometimes your best isn't good enough and there's absolutely nothing you can do about it. Sometimes, no amount of hard work will get you the win. So, what then? Well let's start with the before: do the thing, get through the excruciating waiting period, and then we can move through the after.
Before is the part where you actually do your best and give your all. Let's say you have a big audition (interview, performance, sporting event, speech, test, etc.) coming up and you're terrified. What should you do? Well, obviously, you should work as hard as you can (to a healthy extent)—do the homework, study, practice, prepare, do some psychological visualization, do whatever it is you know you need to do to give yourself the best chance of success.
After you've prepared yourself as much as you can, do the actual thing. Don't worry about feeling ready because a wise man once said "If we wait until we're ready, we'll be waiting for the rest of our lives." You've done the work. You're as prepared as you can possibly be, so just do it because there's nothing more you can do. You cannot do any better than your best. You cannot give any more than your all.
This concept tends to wrack our brains during the agonizing period when we're waiting to find out if we got the part, scored the job, won the competition, made the grade, etc. The truly beautiful thing about it is, while it is stressful to not know if your best was enough, there's nothing you can do about it. Yes, it's terrifying when a part of your life is out of your control, but that's also very calming (at least to me) because while there is nothing you can do to fix the situation. There's also nothing else you can do to mess it up because it's already done. The ball is out of your court and you're waiting to see if it will get thrown back or not; your work is done.
But the after is either the best or worst part of this whole experience. If the outcome is favorable, then your job is pretty much done—you did the hard work and your hard work paid off—go you! Just make sure you don't get cocky. However, if the outcome is unfavorable; then you have to deal with an unbearable amount of self-doubt that is certainly trying to crush you and all your dreams. But, you know what? Just because your best wasn't good enough for this, doesn't mean your best isn't good enough for something else. Just because you fail at one thing, that doesn't mean you will fail at everything. In fact, it doesn't even mean you're bad at the thing you "failed" at—maybe someone else was better or more suited. Don't let that thought get you into the trap of comparing yourself to others, but let it give you perspective about life.
If all humans walked around doing their best all the time, there's no way everyone would succeed all the time, but that doesn't mean that you don't deserve to succeed (or that you deserve to fail). Sometimes we get what we deserve, sometimes we don't. Sometimes it isn't even a matter of deserving at all, but rather a matter of luck. Whether you believe in luck, fate, God's plan, karma, or anything else, believe this: you will not always get what you want, you will not always get what you deserve, but you will always get what you get, and there's no point in throwing a fit because life isn't fair and sometimes your best isn't good enough.
I know that last sentence sounded like I'm pounding you into the dirt, but I'm not trying to sound mean. I'm just trying to give you a realistic (and slightly optimistic) view on reality. I promise I say all this out of love and personal growth that stemmed from experience. Do your best, leave it all on the court, then wait for the response. That way you never have to wonder if you could've done better, you never have to wonder if it was your fault, because you gave everything you have and that's all you can give.