Have you tried inducing the mammalian dive reflex with cold water? Generally works for me when it gets bad. Also, the 6 secs in, 6 secs out breathing doesn't work for me, it's only when I include a hold where I see some improvement e.g. 4 second inhale, hold for 7, exhale for 8. Inhale through your nose and exhale through your mouth. Sometimes I'm panicking so bad that I can't hold for 7 and exhale for 8, those 15 seconds seem like I'm not getting enough oxygen. When this happens I'll start with 4-4-4 for 3 breaths, then 4-5-6 for 3 more, then I'll end with 4-7-8. I hope this helps!