If you are having a hard time breathing (you have difficulty whenever someone tells you to breath deeply or focus on your breath) one solution is to focus on moving your spine and ribcage while you breath.
I often deal with yoga students whose breathing is very shallow or they gasp or begin their inhales with a sharp intake of breath.
My remedy is to have them not focus on their breath but rather the movements of the body that can cause inhales and exhales to happen. And so I teach them how to breathe by not focusing on their breath but by focusing on their body instead.
Often when doing difficult yoga poses or intense stretches I have my students breathe into their nose and out of their mouth. It can feel pleasant to purse the lips while exhaling.
Another method is to hum or chant a "ha" sound while exhaling. While doing so allow your ribs to vibrate. When doing a "ha" exhale I like to open the mouth fairly wide.
Another breathing technique that can feel good and may help to release stress is to curl the sides of the tongue inwards. Poke the tongue through your lips and breath through the "straw shape" created by your tongue. When exhaling release the tongue and breathe normally through the mouth.
Since the diaphragm is buried deep inside the body, I don't often start with diaphragmatic breathing. While it is an excellent breathing exercise or method, I prefer to focus on moving the spine and ribcage since it helps to mobilize spine and ribcage and help my students to feel these parts of the body. (I know that prior to starting yoga I never really considered my spine and ribcage as flexible entities.)
If you sit on the floor cross legged on in a chair with your back not resting on the seat back you can begin by feeling your tailbone.
Repeat this exercise slowly and smoothly. Focus on feeling your tailbone and lumbar spine. (The lumbar spine is the part of the spine that connects the ribcage to the pelvis.)
Next focus on feeling your thoracic spine, the part of the spine to which the ribs attach.
Note that sinking the chest downward is important. It can feel like you are being a couch potato but that is the goal. Then as you bend your spine backwards starting from this position you get a bigger range of motion. It is then easier to feel (and enjoy) this movement.
Next do the same movement but focus on your ribs.
As you get comfortable with the movements, then begin to notice your breath. Don't focus on your breath, just notice what happens as you lift your ribs and as you sink them down.
These movements should be naturally creating your inhales and exhales.
To make your inhales and exhales deeper try to make the movements bigger.
Next pay attention to the sound of your breath.
Note that lifting the chest is not the same as lifting the chin. I have some students who have difficulty with this breathing exercise to begin with and as they lift their chest they lift their chin.
Once you have the feeling for lifting your ribs and bending your spine (while inhaling) you can add a chin tuck or focus on making the back of your neck feel "open".
(This is actually a good way to practice eliminating "Forward Head Posture".)
I usually instruct "pull the ears back and up" or you could sit with your back against a wall and reach the back of your head up and back to touch the wall while opening your chest.
You can then carry this same breathing method into breathing yoga postures. These are like some regular yoga postures except instead of staying still you move while holding these postures. The movements are exactly the same as what you have just been practicing.
The most important thing is to make this breathing exercise feel good. You may find that the slower and smoother you do the movements the better they feel. And then you are ideally no longer having a hard time breathing.