What I've noticed helps most are distractions. I've started playing video games that take up all my attention, and feel great while I'm playing. I know exactly what you're talking about though. I've had nightmares and woke up with my heart racing, and then the breathing thing starts. It's such a weird feeling, but if you think to yourself that even though it "feels" like you can't breathe, yet you can, then you're okay. You are breathing, so you're okay, so it's okay. Try to think of it that way and see if it helps! There have been days upon days that I've "felt" like I can't breathe, but I keep breathing! Anyway, it's actually been a lot better since I've been thinking of it that way! Hope you feel better soon!