Like many others, I lost my SO and best friend a couple weeks ago. However, since this is sort of open ended as to what you'd do without your best friend, I'm going to answer it in terms of my best girl friend.
I've known the woman for 16 years. When we met, I was riding down the street with no shirt on. She thought I was a boy. (Easy kids, I was 3). She was raking leaves. From then on, we've been inseperable. I swear, she knows me better than I know myself. She knows what I want, knows what I need, and drives me further to achieve what I want to do. She'd had some really hard times in her life, struggling with clinical depression, but I like to think we got through it together and made our friendship even stronger. I don't know what I'd do if I didn't have a best friend like her, and I can safely say that we'll be friends for the rest of our lives. I love that girl