Hey there B.....do you have a Stamp book? I would do pages 10-11 non-stop (keeping your horn on the face and breathing through your nose) for as long as you can with the sole purpose of keeping your jaw where you need it. See how long you can go before your "foundation crumbles". Do it again the next time you practice and see how much further you can go. With consistent practice over time, your muscles will memorize where they should be and your foundation crumbling will go away. Also, many method books will work for this (Clarke, Arban). It is up to you to make the concentrated effort of keeping your horn up and jaw out for as long as possible to build up the endurance.
(Pardon me for jumping in before you Manny......
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