How can I get myself motivated so I want to keep on writing?

[Linda]

I've tried to write a book... I don't know how many times. But I've always failed, stopped after a few thousand words, either losing interest in writing (but that only lasts for a few weeks) or getting a new idea that I like more than the book that I am then writing.
You see, I have a great idea for a book, and I've already written a little more than 13,000 words. But, you see, I feel myself slowly losing interest in writing it, longing for starting a new book. But still, in my heart, my dream is to publish this book. I am proud of it, and I want to get it completed on paper.
How can I motivate myself so I will WANT to open up my computer and keep writing, instead of losing the interest in it before I even manage to open up the document that holds my story and just do something else, like check my Facebook page or e-mail.
This is my biggest dream, but I need something to motivate me so I can accomplish it.
Have you been in this situation? What would you do, if you were me?
Thank you so, so much for your help! I really appreciate it!
~Merry Christmas~