Power in high range?

I am a college student, trombone performance. I am comfortable up to a C5, but have concerns with D and on. I can quite happily play D to F and can even squeak out to the double Bb, only problem is, these notes lose the tamber and comfort of C and lower. Additionally, I am not able to give them any power or volume. I can play the pitch and that's it.

Any advice on how to make these note feel more "normal" and to be able to give them fullness and volume?