This is part of my daily routine. My panic attacks take a different form, but this seems to help.

You just sit comfortably somewhere and count your exhalations silently. Inhale, ONE, inhale, TWO, inhale, THREE etc until you have completed thirty six breaths. If you get lost on the count, start over. If you can't keep track of the count for that long, you can do four blocks of 9, at one time or throughout the day. Or 9 blocks of 4 if that is all you can keep track of. You will get better at it as you go along. Allow your breathing to occur naturally. Over time, your breathing will become easier, deeper and more rhythmic throughout the day, and I would expect breathing attacks to gradually taper off. Mental chatter should also subside.

Sorry you are going through this. Panic attacks can be a real drag. My last one was tachycardia - I had a pulse of 191 for a period of about 10 minutes. Like you, I went to the ER and spent a $100 co-pay to confirm that there was nothing wrong with my heart.