I'm a third year brass student at the Melbourne Conservatorium. How do you stay motivated to work on something in your basic setup like the turnaround of breathing in to breathing out? It is such an important part of all playing, that it is tricky to move forward if I can't get the turn around how I want. When do you decide to move on when you cant work through things and come back to them later?
Whilst some problems can be resolved, many factors in searching for excellence are things that need constant work over many years. Think about incorporating work on the problem into your daily routine rather than having the idea you can fix it.
As far as the problem itself, consider the idea of movement of air as being circular. This will get you away from feeling a stopping point at either end of the breath and hopefully smooth things out for you. Good luck!