Annoyance

So, I've been trying to work on my book, Portals, ever since I got home. It's definitely alluding me. The problem is that I have begun yet another book. This one is annoying because I don't know where it's headed. It just started writing itself in my head. So I had to get it down on paper. I've found that unless I get it down on paper, it keeps pounding in my head until I either go insane, or I write it down. But if I wait too long, it's not as good as if I write it right then. So I write.
I have a goal of finishing Portals by the end of the year and I would really like to send it into a publisher and see if I can get it published, but if I can't write it, I can't send it. So that's my annoyance at my brain.
Any ideas on how to help me would be much appreciated. :)