I don’t understand why it’s important to experience a full diaphragmatic breathing when you’re doing anapanasati if the goal is to develop focus and mindfulness. The task is to attend to the nature of the breath, body, and attention – to just observe – and the last thing you would do is to manipulate your natural breathing.
Then again, I don’t know whose instruction you’re following. Maybe you’re doing something entirely different or for different purposes.
In all, I don’t think you should worry so much about nailing down a perfect breath. Maybe you’ve always breathed shallow but you never noticed. Maybe one day you’ll start paying attention to different breathings throughout the day. Maybe it’ll grow deeper and more comfortable. Maybe you’ll discover a link between shallow breathing and its cause, or maybe not. The important thing is to sharpen your observation. If you pay attention, you’ll make discoveries out of that 40-minute session. For starter, where does your breath begin? Where does it end?
In my experience, some days my breath felt stuck above the collarbone; some days it flowed into my throat; some days I felt it pushing my rib cage. For one, every day is different. The more you sit, the more sensitive you’ll become to your breathing. For another, I don’t believe that the breath is always naturally long and full. It just echoes your state of mind, no? If you’re resisting something, unknowingly you become restless, and the breath would reflect that. But that’s just my own body wisdom. I’m not supposed to frame anything for you. I learned mine the hard way, a lot of surprising emotions, all the while my meditation teacher simply said, “Oh.”
You might find the instruction in this book helpful. Here’s a classic though a little harder to read.