Fear of change is an absolutely natural and healthy response. Sometimes fear is reasonable, sometimes, not so much.
In your case (just some examples):
- fear is reasonable if you know that you cannot do the job, and the chance of failure is high.
Solution: before taking the new position, make sure that you learn how to do the job. Talk to your boss about it.
- fear is not so reasonable: you know that you can do the job, but you do not have the experience.
Solution: talk to your boss about it. Whenever you do something (important), have it reviewed by somebody else (maybe even by more people - it depends on the situation), write a short report about the review contents and the decisions taken (and share the report with all the participants immediately after the meeting).
In this way, you share the responsibility with others in an official way - the way it should actually be.
Has any of you some afraids of making terrible mistakes?
As I said, fear is natural and good - when done reasonably. And there is no human being, alive and working, to never do any mistakes. One just needs to prepare, to minimize the number of mistakes, and their magnitude.
Any tips when making hard decisions as a high job position?
Decisions are decisions. You analyze the data, see what works best. If nothing works, try to find another alternate solution which works.