Hi Adam-
The good news is that what you describe is not uncommon--not at all. There's not a writer I know who's immune to this problem, myself included. But I'm going to share a little secret with you that a very wise writer taught me a long time ago: lack of motivation comes from inactivity. This is true with almost everything, and it applies to writing as well--especially writing. The fact is, the less you write, the less you want to write. How to overcome this? Make it your goal to write something each and every day, even if it kills you. It doesn't have be a hundred pages--it doesn't even have to be a full page, so long as you write something. Even a paragraph is fine. Even writing about the fact that you don't want to write. Something. If you do this you'll find the writing process becomes much easier and--surprise--even enjoyable. I know it almost sounds too simple to be true, but trust me, it works. It's how I manage to complete my novels in just a year. ) One page per day=365 pages= 1 complete novel!)
Give it a try and see what happens.
Best of luck,
Andrew E. Kaufman
http://www.andrewekaufman.com