This happens to me all the time, so take solace in knowing you're not the only one who gets like this.
This is one of the rare times I'll say and admit that ignorance is bliss. It really, truly is, as sad as that is to say.
The reason you fear 'blasting off', if your anything like me, is that you know just how powerful the experience can be. There is no telling what to expect, and no matter how many times I safely come down every single time I dose I feel like its going to be the last time. Each time I feel like I'm one inch closer to death... that one of these days I just won't come down.
If, when you read those words, you feel the same way somewhere deep down inside, then I must, MUST express to you the following: this is normal, to be expected, and you do not need it at this current moment in time. I say that because if you have any feeling of holding back, its a sign that you need to work something out in the here and now present reality before you make such a giant leap into 'other world'. There is always a measure of anxiety involved with taking a psychedelic, but if you are having genuine fears about it then now is not the time.
I've taken a three month break from psychedelics at this point, and there have been several times I've thought about dosing, but I'm always steered away from it for whatever reason. And, just about every time I do, I get this intuitive feeling like it was the right decision not to journey. It shouldn't be such a scary thing if your really ready for the next step, the next lesson.
Or, at least that's my personal opinion and the conclusions I've drawn about myself and why I fear the substance so much sometimes. I really hope that added some perspective, and if and when you do journey, have a safe voyage my friend!