Well one of my students has already given you excellent advice.

Learn to relax and buzz loosely first. Or try "spitting seeds" and let the air continue.

The reason for your problem is that you need to learn to focus the buzz.
 
The mouthpiece rim creates mouthcorners. It does not allow the outside portion of the lips to affect the part inside the rim. (Very little anyway)  This dependence on the mouthpiece to make your corners has kept you from developing you lip muscle fully.

When lip buzzing my corners lock down inside the area where the mouthpiece cup would be. They do this without the aid of the rim because I practiced to allow them to develop this. I can lip buzz a note and bring the horn up and play the note. There is not an octave jump.

This lip buzzing skill is valuable in obtaining the extreme upper register and in connecting the registers. Nobody has this skill until after they practice it.

Also lip buzz the same embouchure that you play. This is where most people go astray. They buzz with their lips set at a different angle than they have them when they play. When lip buzzing the lips are able to move into the position that they want. Lots of people created embouchures instead of allowing the lips to find the set that they buzz best at. This is why so many resort to pressure and other bad habits.

If you are currently playing and need to continue to use the old embouchure at gigs that is fine. This will not destroy the old embouchure. It WILL build a stronger new embouchure.

Information about my 3 books. "The No Nonsense Trumpet From A-Z" , "Trumpet FAQ's"  & "The Next Level"  http://www.BbTrumpet.com
 
Best wishes
Clint 'Pops' McLaughlin