I am glad I found this site. I have had this problem for a while and it terrified me. It's sometimes accompanied by severe dizziness and disorientation, chest pain, a tight feeling in the chest or laboured breathing...I have been told that all of these things can be caused by anxiety, I usually get them at separate times, but when I get them all at once it is really quite unbearable...I have tried taking valium(diazepam) but often with little to no immediate results once it gets to this stage...
but it does definitely help. But I would not recommend this if it is a common recurring thing as it's not a drug that should be taken too often.
It is very calming to know that I am not alone and that this seems to be a fairly common condition. I was really freaking out when it first happened.
It doesn't stop as soon as I manage to calm myself mentally, but if I am able to relax myself by focusing on relaxing my muscles and feeling the tension fade away, it makes it a lot easier. It also REALLY helps if I can distract my mind somehow, by getting involved in a book or videogame or movie and trying to calm and focus my mind on something else, it eventually fades until I stop and think about it again.
The key is to not be scared of it happening or think about it if you can avoid it. I know this is a LOT easier said than done when you feel like you are suffocating, but it really is the only way to get past it that I have found.
I hope this helps, I know how terrible it can feel to have this happen.