High notes or developing range isn't hard. 1)When you play, the oral cavity adjusts to the note you are playing. To show what I'm talking about, pick up your trumpet and play a low C. Now play the A that is your highest note. Notice that the oral cavity adjusts to the different notes? If not then maybe you are not adjusting your oral cavity. 2)Can you whistle? (Even if you do it poorly) I sure hope so! Whistle a scale. Notice how your tongue adjusts to the different notes? This is what I mean by adjusting the oral cavity. 3)Now, pick up your trumpet again and when you play, be aware of how your oral cavity adjusts to the notes you play. I'm guessing that your situation is just a matter of learning how to adjust the oral cavity. I tell students when they do lip slurs, think of them as tongue slurs since the tongue plays a major roll in this process. A site I've often referred people to on this topic is a site called "Arch Tongue and Hiss" Once you get the hang of it you'll say "Wow!! High notes aren't hard!!"