In this world, magic comes from a large tradition of stories. There is a tome of stories (or some such thing) that holds all of the stories that a particular culture knows, and this is passed down to the story tellers, who are able to perform the magic.
The concept is that, when facing a particular situation, a story teller will tell the story they know, modified to fit the current facts. For example, if there is a story of a young girl who grows so large that she can walk across a lake, then an old man telling the story in order to get to his cottage on an island would change the details so that it is him who is doing the growing and walking. In so doing, when the story is done being told, the old man is the one who crossed the lake.
The key is that in order to make the magic work, you have to believe that the story is possible.
The problem I'm having is that the plot's central moment will be when a new story teller, who has yet to actually perform the magic, suddenly realizes that it's not about believing the actual myth, but about believing that you can do what the people in the stories did, and weaving the magic to make that happen.
So...why doesn't everyone just write their own stories whenever they need them?
Why is it important that there are only so many stories, and what is so special about these stories that they are the ones that invoke the magic?