Mind cannot be still. It needs continuous thinking, worrying. the mind functions like a bicycle, if you go on peddling it, it continues. The moment you stop the peddling you are going to fall down. Mind is a two wheeled vehicle just like a bicycle and your thinking is a constant peddling. Even sometimes if you are a little bit silent you immediately start worrying, why am I silent? anything will do to create worrying, thinking, because the mind can exist only in one way - in running. Running after something, or running from something but always running. In the running is the mind. the moment you stop, the mind disappear.
Right now you are identified with the mind. you think you are it. From there comes the fear. If you are identified with the mind, naturally if the mind stops you are finished, you are no more. And you don't know anything beyond the mind.
The reality is you are not the mind. You are something beyond the mind. Hence it is absolutely necessary that the mind stop, so that for the first time you can know that you are not the mind..... because you are still there. The mind is gone; you are still there - and with greater joy, greater glory, greater light, greater consciousness and greater being. The mind was pretending..... and you had fallen into the trap.
What you have to understand is the process of identification - how one can get identified with something that one is not. Remember the Lion cub which was raised with sheets behaved as sheep till he saw its reflection one day in the river and suddenly he roared like a lion. Its identification as sheep was gone and he realised its true potential.
Your mind is not created by nature. Always try to keep the distinction: your brain is created by nature. your brain is the mechanism that belongs to the body, but your mind is created by the society which you live - by the religion, by the church, by the ideology that your parents followed, by the educational system that you were taught in, by all kinds of things.