I do not know why the certainity of your faith is different than a certainity of anything else in your life and why its a different type of predestination for you.
Your birth was predestined, your death is predestined, your day tomorrow is predestined, also your salvation is predestined.
Not sure what you're saying about the certainty of my faith? I am certain that I am saved because whosoever shall call upon Him shall be saved. And I've called out to Him.
As far as predestination here's a Scripture about it:
Romans 8:29 For those God foreknew, He also predestined to be conformed to the image of His Son, so that He would be the firstborn among many brothers.
Those God foreknew (He knows the end from the beginning). He predestined me to be conformed (transformed) into the image of His Son. So that He would have many brothers.
His predestination is that I would be a son of God. And that I would grow in knowledge of Him. This is the work of Holy Spirit. He is my Teacher. He trains me up in righteousness.