Bilateral Breathing - Again

I've been reading past posts and threads on breathing bilaterally. We have a wealth of information here on the pros and cons. I've decided I want to breathe to both sides, but am having some problems getting there. When I'm fresh, I think I'm doing ok. I feel like I'm rolling enough that I'm not turning my head too much and grabbing a nice breath, weak and strong side alike. When I begin to tire, my weak side breathing falls apart. I feel like I'm sinking - I know I am, because I'm lifting my head up to catch a quick unsatisfying breath that's half water. I can feel that my arm is dropping as I'm pushing my head out of the water. I know my hips are sinking too: I noticed when I did a short set with a pull-bouy today after tiring, I felt like I had all the time in the world to breath on my weak side. I would like your thoughts on (1) what is breaking down on me, where the weaknesses are and (2) other than keeping at it with the hopes of getting stronger, is there anything specifically I can attend to or any drills to help me isolate the problems. Even if you just tell me that the struglle is all part of the process, I'll be grateful for any feedback you can provide. Many thanks - Barb