How should I deal with this?

My bestfriend is one of the best girls. She was there to hold me while I cried when I found out my parents were divorcing. She's always had my back, taken my side. She's always been there to look out for me like a sister. However, she has one MAJOR flaw - she hardly ever lets me get a word in edgewise. I'll be telling her a story, and she'll cut in and start talking. When she takes a break, I'll say "You never let me finish my story." and she'll say "But wait, let me finish!" When she finishes one story and I try to start talking, she'll launch into some other story. By the time she stops talking for good, I've forgotten what I had to say. The last time she did this was when, on the way back from a trip to Atlanta with her family, I was trying to tell her dad about a movie I love. I asked if he'd seen it, and he said no. I'd gotten as far as "It's about a man and his wife who are really poor, and their son has a heart failure while he's playing baseball. They don't have they money to cover the surgery he needs, and -" and she busted in with "Their insurance doesn't cover it!" I looked at her and said, "You know I hate that, it's MY story." She said "But you didn't tell them about the insurance!" "I haven't gotten there yet," I said. I continued my story. About a sentence and a half later, she busted in with some other detail I hadn't gotten to yet. She did this quite a few times, with me only getting about ten words in at a time, and each time I asked her if she would please stop and let me tell my story. After about the tenth interruption, I finally got to the point where I just threw a tantrum. I screamed at the top of my lungs, then said, "Oh my Lord, PLEASE. SHUT. UP!!" then refused to tell my story. I hate to be rude but I'm getting tired of her always interrupting me, especially when I'm telling a story. Her dad even told her she was being rude. How can I get her to stop cutting me off so often?