I often say as a joke that if someone's evening is ruined by me missing a note or two in the concert, I'll gladly reimburse his ticket!
Seriously, I use to be upset after missing a note in a concert or even in rehearsal. With time, I learned to accept that I'm not a machine and that missed notes are just part of the show. Of course, no one misses notes on purpose but if you just focus on that, you get more nervous about it and end up missing more.
There are a few things you can do to improve your accuracy though. The most important is to hear the notes clearly in your head before you play them. Try singing your music with your voice before playing. Take your ear training class seriously! We often say, if you can sing it, you can play it.
Being in good shape and having a good feeling in your chops will also help you. If you get tense, you'll miss more notes. That's why I said earlier that you shouldn't worry about it too much. You'll end up in a vicious circle of worrying-getting stiff-missing more-worrying more etc.
Be patient when you learn a new piece. Your brain and body need time to develop the right reflexes for each new piece or etude you play. If you allow yourself proper time and don't expect perfection immediately, you'll improve gradually and will gradually miss fewer notes.
When I go to a concert, I want to hear great music, musicality and energy from the players. I don't really care if they miss a bit as long as they try to make something interesting. I tell my students not to try making something perfect but something beautiful. The purpose of music is not to hit all the notes but to touch people's hearts.
Next time you practice, free yourself from the obligation of hitting all the notes. You'll still miss a couple but you'll sound much better, your chops will feel great and you'll have a lot more fun playing.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.