Expectations

Someone once told me that motivation and expectations are like a hammer ; if you have a big one, your nail will go in faster but if you hit your finger, it'll hurt a lot more!
A lot of the stress we feel when we play comes from the expectations we have about our playing. If you set the bar so high that you can never reach your goal, you set yourself for a lot of tension and frustration. Also, if you've never achieved something in your practice room, it is unlikely that you'll achieve it on stage. 

We often hear that you need to aim for perfection if you want to achieve excellence. It may be true in some areas but in brass playing, this approach might cause you more trouble than anything else as it will make you tense and nervous. You just won't have fun playing if you don't give yourself a chance to be human. 

On the other hand, if you don't care at all,  you won't practice and will not become a good player. You don't want that!

The best is to fix yourself ambitious but realistic short term targets for your playing and achieve these goals. That way you'll always have small victories and will be motivated to practice without the frustration of failing all the time. Once you've reach your target, set the bar slightly higher. Be ready to take a few steps back if you feel you aren't making progress anymore. We all have ups and downs so don't get mad if things don't go your way all the time! 

For performances, you can expect something a bit above the average of what you normally do.  If you have a chance to perform the same piece again, try to be a bit better than the previous time you played it. 

It can be mentally difficult to accept lower expectations. and it takes some courage to let go of some of our perfectionism. 
LOWER EXPECTATIONS DOESN'T MEAN NO EXPECTATIONS OR LOW EXPECTATIONS.
You can still aim quite high even if it's not perfection.  It'll just be more realistic and you will sound better, have more endurance and play more musically at the end.