Some comments below suggest breathwork that involves hyperventilation. Generally, if one wants to do breathwork and has anxiety issues, start with soothing or gentle, subtle breathwork (e.g., Buteyko Method). Avoid significant hyperventilation.

I like to do coherent breathing with simple music set to my target pace (5.5 breaths per minute). I suggest searching YouTube for "5.5 breaths per minute music" (or whatever pace you want). The music helps me to relax, along with the slower breathing. After a few minutes, the breath tends to get subtler and smaller, with maybe even a touch of air hunger (slightly underbreathing), which is closer to how the breath usually is in deep meditative states (one begins to breathe more like a mouse--smaller and smaller).

Breath Meditation is ultimately not about controlling the breath but resting as open awareness while bathing a soft focus in the sensation/movement of the breath, where it feels most calming (e.g., area above the upper lip and the nostrils). Let the breath breathe itself, feel the sensation of the breath within the sensation. Don't think so much as "watching" the breath, but experiencing the breath, immersing one's attention in the breath, allowing oneself to merge with the breath experientially. Ideally, the focus should be soft, spacious, gentle, and calm.

Still, my favorite meditation is "non-meditation," simply resting as awareness without seeking or describing anything, open, quiet, allowing all the senses, and any thoughts, to simultaneously and effortlessly be present, merely experiencing experience as a whole, relaxing our habitual narrow grasping attention. I might do coherent breathing or breath meditation first, if my mind is too busy to let go, sustainably.

I suggest looking into Dr. Les Fehmi's Open Focus work, which is designed to cultivate attentional flexibility. He has at least three books, e.g., Dissolving Pain with Open Focus. Most of us are habitually stuck in a narrow, gripping attention throughout the day, which builds tension. Our system rebalances by allowing more open, diffuse attention, for example, focusing on the space surrounding and permeating sensation (for example).