I think jkircoff said it best. The fix is going to be plenty of patient practice time. When I started college as a freshman I went from whatever smallish mouthpiece came with the King 2280 I played in high school to a 51D, which really didn't feel that extreme, and I liked it. My teacher suggested to me after 6 months or so that I should consider switching to a larger mouthpiece, and I played on a Wick SM3.5 for the rest of my college. When I made that switch I lost some of my upper register and I was missing partials a lot. But with time and practice, it got better.

I currently play on a Wick 3AL for euph which feels really nice to me. I thought it would be fun to learn some trombone, so I bought a pBone and a Kelly 6 1/2 AL. At first, I couldn't believe how small the 6 1/2 AL felt and I really had trouble anything lower than a D in the staff. But after a year or so of practice with long tones and simple melodies on the trombone, I can switch between the 6 1/2 AL and the 3AL with no problems after a little mouthpiece buzzing and long tones to reorient myself to what's on my face.

Don't worry, just give it time and lots of practice!