I too have been through breathing issues with anxiety. It is very scary! I found that walking outside (no matter the time or weather) makes a big difference, especially if I concentrate on finding a specific flower, or bug or a branch that looks a certain way, whatever pops into my head is what I look for.
When DD7 gets super upset about something I sometimes sing a lullaby to her that I use to sing to her when she was a baby. It calms her down almost instantly! I am amazed that at almost 8 she will still respond the same way to a silly song I made up using her first and middle name when she was a crying month old baby in the middle of the night! (Even more amazed that I remember the words and tune

) As she has gotten older we have had less cases of her getting overwhelmingly upset, but there are still occasions when she is just gone and the song works best. She even starts humming it herself as she calms down.