So, I've been playing horn for about four months, and I think I'm pretty decent at it. Recently, I got first chair in my high school's intermediate band. I have pretty good range for a starter, maybe up to A or B above the staff, and decent tone as well. But, I'm encountering some problems. Occasionally, instead of improving, I kinda become worse, and I have to rebuild my embouchure to what it was before. For example, if I could play B above the staff and low A below low C, slur easily, and have good endurance, the next day, I'll have trouble hitting a G above the staff or a low D, have trouble slurring anything above C, and have horrible endurance. Even when I rest, it doesn't go back to normal, and it takes me around 5 days to get back to normal. This happens every two months or so. I don't really understand why this happens because I practice every single day for at least 45 minutes, and I'm pretty sure I play with correct technique. It's really discouraging for me, and even when I want to get better, I can't. Can anyone help me on what I could do different to prevent this kind of stuff and what to do better? Thanks.

just keep practicing. It can be discouraging, but u'll get better... look up youtube videos about french horn. It takes time trust me when I started I could barely play without sounding like Chewbacca. Now less than 2 years later I'm in my schools wind ensemble as one of the top horn players in the program.

Do you keep a practice log? If not, you should definitely start keeping track of when, what, how long, and how you feel when you practice.

Basically, I recommend you use the scientific method on this problem to figure it out on your own. Come up with a hypothesis (doing [x] will make things better/worse) and an experiment (your practice sessions and rehearsals that you keep a close log of) to test your hypothesis. The experiment isn't controlled well enough to publish in any scientific journal, but you can still get personal results.

Your description of the problems you encounter suggests to me that you are using too much lip pressure onto the mouthpiece and this is leading to fatigue and a lack of continuity in your progress as a player. This is not dissimilar to something I used to suffer from (about a hundred years ago it seems) but I was put on the right track by my tutor at the time, Ifor James. He first showed me some exercises to help develop my lung power/breath support - his was phenomenal, so much so that when he released his waistcoat (I think it's called a vest in the USA) his ribcage looked almost deformed because of the massive size of his lungs. The second stage was to concentrate in developing the correct 'buzzing' technique with my lips - he trained me to play every piece of music successfully on just the mouthpiece - this helped to identify the correct technique and led to me using far less pressure when using the complete instrument - and this led to increased stamina which in turn helped me enjoy playing more.