Right before I decided on what I perceive as a memoir or something of an autobiography (the memoir instructor said some of the stuff I had included borders on an autobiography), the idea of a fictional novel has also occurred to me. But when I began writing notes, it sounded more like memoir. There are many other things I could have included but didn't, which would have made the story seem more like an autobiography.
As for creating an autobiographical fictional protagonist, I think I may have done so in the diary novel I have begun. Except that the protagonist is a guy and I'm a girl! But it is set in the 1980s, and that is when I grew up. And the character is dreading having to get braces, something I myself went through then. So in some ways the character is autobiographical, the character's gender notwithstanding.
Is this really what is meant by "write what you know"?