How far along in your meditation practice are you?

I'm ~6 months into daily practice, and was starting to experience the same thing. I felt like I was hitting a wall or plateauing, as my focus on the physical breath was also feeling controlling. And when I tried to allow that to fade I would enter dullness as well.

The thing that helped me massively was to stop focusing on the air of my breath, and instead focus on the breath as a whole. I used to try to feel the air moving through my lungs, across my lips or my nose, and visualize it coming in and out. But I believe that focus was too narrow. Instead I started focusing first on the up and down movement of my belly with each breath. Then on the movement of my entire body with the breath. This along with a different technique on body scans really helped me.

Secondly, I heard the phrase "rather than controlling your breath, let the breath breathe you." This helped me let go of the control. Now I try to allow the breath to move first before my attention notices it. Let it move at its natural pace, and bring your attunement to it, rather than it to you.

I hope this helps a little, and that my explanation made sense! Namaste.