When I have trouble breathing because of anxiety, I lie down and actually pay attention and note that I am actually breathing and use "logic" to help calm me down -- if you are breathing at all, you're still alive (so there's still "hope"/you are still "all right").

Just note the breath in/breath out and work it like it is an exercise you're supposed to do, as if you were working with any other part of your body. Experiment a bit and see how long you can hold your breath, how long you can exhale before inhaling again, etc. Get "clinical" instead of "personal" about it and thus use your head to help your body relax and do its thing better.