I'd agree with indre and ezjim. The first time you had a bad trip maybe you had some stress or were going through something. Taking psychedelics if your depressed or stressed out in any way is not good. I learned that the hard way. Now, you're probably so worried about having a bad trip that your worries are self fulfilling.

I used to eat a lot of mushrooms, pretty much weekly for 2-3 years. Then I had a really, really, really bad trip on 1/4 of shrooms shortly after breaking up with my live-in girlfriend. That was 10 years ago and I haven't had any psychedlics since then. I'd love to but just know that I'll have a bad trip simply because I'm worried it will happen. I never want to relive that trip so, sadly, no more psychedlics for me.

Since your still trying it sounds like your bad trips, fortunately, weren't as bad as mine was so you still have hope. Try telling yourself that it's only happening because your making it happen and if you can subconciously make yourself have a bad trip you can subconciously make yourself have a good trip too. With any luck your next trip will be as fun as your earlier trips.