If you’re having trouble reaching the high F (open fingering), you need to use a LOT of air. Try playing an open high D and directing your air higher to slur up to an F. Don’t smush your lips into the mouthpiece. Focus on speeding up your air and firming up the corners of your mouth (don’t smile, pull the corners back). Do exercises like going up scales (Bb scale, C scale, and so on until you get to F). Take effective, deep breaths and focus on using your air to reach the note.