Faith in itself has no value. It’s only the object of the faith that has value.
Suppose that I promised you, “I just put $100 in your pocket.” Now, are you sure you have $100 in you pocket? Well, it depends upon whether you believe me or not. Do you have faith in me? i.e., belief without seeing? Well, if I’m reliable, you do have faith in me, and you know you have the $100. However, if you can’t depend upon me, then you won’t believe it. In fact, even if you do have faith in me, but I’m not reliable, then you don’t have the $100 even if you believe that you do. It all depends upon me, the object of your faith.
The same is true of salvation. My faith in Christ has value in what Christ did for us on the cross, not in my act of faith.
The same is also true of assurance. If you believe, you’ll be sure. If you don’t, you won’t.