Hey! so I used to have the same problem as you. What I did was I played my highest note that I could play and I focused on making my air go faster. Don't try to add pressure to your lips when you blow faster air since it'll stop your lips from vibrating. The reason why you can't go high is because you most likely are putting a lot of pressure on your mouthpiece when you attempt to play high notes. Try to not put a lot of pressure and just focus on creating the sound. :)