Our earliest beliefs came from our parents.
As we got older our beliefs about ourselves, stuttering, other people and the world we live in, were created though our experiences.
The good thing is, that beliefs are an ‘inside job’ (created in our minds) about the external world. They are not reality in the sense of an object like a ball which you can hold in your hand.
If you believe you can do something or not, it doesn’t make a difference either way.
First you have to tell yourself whether you can do it or not, before you can play it out in real life.
If this holds true, then why not just tell yourself you can do it, and give yourself a chance from the start?