I can't really know because I will never meet you and I'm not an expert, but it sounds like you're really anxious about having sex. So much so you're giving yourself an anxiety attack or, on the other hand, getting so excited about it you're not breathing properly. Sex, like most things, goes best when both parties are relaxed with it. That comes with experience -- we're all nervous about it when we first start doing it. That goes away, or at least it does for most of us. So I'd ask, are you anxious about other things as well, or just sex? Talking it out with someone older might help. If it's an overall anxiety problem, a therapist can help. Eventually, sex will become a normal thing and you won't be nervous about it anymore, but if you are getting this nervous or this over-excited, I do wonder if you have other parts of your life that are also made more difficult by anxiety. If so, then anxiety is your problem, not sex. Peace.
I agree with Paxiled. You seem really anxious about sex, or at least, are so excited you're forgetting to breathe through it.
Only you know if it's anxiety or arousal, but be honest with yourself about it. Sex can be scary, even after you've had it a bunch of times, but like Pax said, if anxiety is a an overall problem for you, counseling can really help.
Have you and your girlfriend talked about sex? Talking about it may help you relax. You can help plan for things, like pregnancy prevention and disease prevention. Talk about your fears or any concerns you may have, and that may help.
But I really agree with Pax - if anxiety is an overall problem for you, there is help available, and you deserve to get it.