This is your friend, right?
Talk to him like nothing happened. Don't bring it up. If he brings it up and acts aggravated towards you for it, Apologize and ask him for advice. Show him that you know what caused it (stage fright in essence), and show that you don't know how to fix it. If you present to him a situation in which this whole accident can repeat himself, then the only thing he can do is either shrug you off, yell at you more, or help you. If he thinks you're joking, make it obvious that you're not. Give him other examples of you messing up during performances.
Stage fright isn't rare or anything. Sure, some people don't have it. It's not rare in any way, shape, or form, though. A lot of people get nervous before concerts. However, people usually get over this at some point after either performing in front of crowds repeatedly to the point where they don't even notice it, or after learning some trick to get over their fear. I'd tell you mine, but I haven't gotten over it completely myself.
I have, however, had experiences with things that have lowered stress tremendously and have in general lowered my mistakes in performances.
First off, go in knowing that you'll probably make a mistake. It's a live concert. It doesn't have to be perfect. It won't be. Stressing out to make it perfect will ruin it way more than going into it knowing you'll make a few minor errors.
Second... This next one only applies if you have a family that has been to your performances before, and has enjoyed them thoroughly... If this is the case, look for them in the crowd before the performance. Play for them, not for the rest of the crowd. They're going to like it regardless of what you're doing on stage. You could be tap dancing and they would be laughing and enjoying the performance. It doesn't matter what you do if they're going to love it regardless. I'm not saying that you should just play whatever... but your parents will love the performance regardless! So focus on them! If your parents aren't going, replace it with anyone else that fits this. I'm sure that there is at least one person that you can think of when it really comes down to it.
Those were the two things that lowered my mistakes in performances. I hope at least one of them can apply to you to help somewhat.
There are a lot of musicians on the internet. Talk to a few of them and ask how they got over it. I'm pretty sure you could make a blog on here titled "Attention Musicians: Seeking Help" and you would get at least a few responses.
I'm assuming you're in a college or university with these problems. Sorry that I can't really help with that. Still only a Junior in high school :/. I don't know how intense performances really are in college. However, I do know that in general, not only are you going to make mistakes, but you're going to have bad performances as well. This is just one of those bad performances. The best way to recover from it is to just realize that it was a bad performance. Look at the day of your next performance, see how far away it is from now, and practice your music like no other. If you go in confident that you'll play your music, it usually ends in better results.
I hope this is helping somewhat @_@'. Sorry if I'm repeating things as well. Really tired haha.
Good luck, sir! I hope things go well!
\KILLER/