Need help with completing stories, article etc......

For years I've struggled with actually completing anything I write. Right now I have over 20 topics I want to write about for this blog but when I sit down to write it, I always start imagining the rest of the post before I even write the first sentence. I've tried making really detailed outlines before I write but the same thing happens. I start writing then I think of ideas about what ever I'm writing about and then I look up and 30 Minutes have passed. It's like my mind gets ahead of me and my hand can't keep up, it's bit hard for me to explain but I was wondering if anyone else has this problem? And if any of you know how to fix it.