You have to have the right mentality on it. Once that first thought gets in your head, you're pretty much convinced whatever you want to be convinced. Try to convince yourself that nothing is going to happen and you are having great time rather than a horrible time. Then again, some people just aren't for weed. From your post it seems like you smoked before that, but then again if one bad experience is turning you away from it then I'm not so sure. Smoke less at a time and just chill. If you haven't smoked in a while and aren't sure if you're overdoing yourself, just take one bong rip and chill. If you feel you aren't high enough and want to smoke more, go ahead. If you feel you've had enough to have an enjoyable experience, then stay at that.