Don't smoke so much at once? Get a bit high, get used to the feeling, over time you can smoke more and the little anxiety attacks will be a non issue. It's not that we don't get them, it's just that we realise that its a stupid thing to worry about and worrying about it only magnifies the problem because it's all in your head. Sit back and relax. But until you're ok with that feeling, just smoke less, or slower. Don't just jam a massive nug in the pipe or roll yourself a massive reefer or something.