Don’t get discouraged! trying and failing is an essential part of learning. You have great musical ability and have improved significantly overall in the last couple of months. Keep working at it!
Use good posture! remember to sit at the edge of your chair with your feet flat on the ground. Try to take in deep breaths, through your mouth.
As part of your warmup/ practice routine play a C major arpeggio (middle C, first line E, second line G). keep the air moving forward through the notes, picture the driving through the rainbow item cubes in Mario Kart. Continue using long tones in your warmup.
Work on your music from school, pay special attention to the quality of repeated articulation in “Common Denominator”
Here is the opening horn line from Schubert Nine, first in C (so you can sing it or play in on a piano to get in in your ear) then in F to play on the horn (the one starting on written G). Listen to a recording then start looking at this! Bellow this is a fingering chart.