Ya know, I think this is a really interesting question. To be honest with you, I have never thought of it this way. I do not believe that it is an autobiography. But, maybe there are parts of it that are relevant to her life. She has always said that it was started because of a dream she had. Maybe she added herself in as Bella? Maybe it was about a life she wished she had. Maybe the Jacob part was about a friend she once had.
Sometimes authors write about things they know. Parts of it could be true and about her. When it comes to the vampire part, you never know! We hardly understand the world as it is. The knowledge we have is always changing. Maybe vampires are real. You never know!