Oh man I'm with you. I think its natural to be a mental channel surfer. Ya know once you start a project it's all new fresh and exciting. The closer the project comes to completion the more it seems like work. The drive fades some and our energy get distracted by a "new" fresh project.
I does not seem to matter if it's song writing, planting a garden or building a home. Breaking a project into smaller accomplishable goals is a trick I use but with limited success.
It took me four months to install my dishwasher the wife bought.
Fortunatly, songs, for me, just pop out of no where. To be honest everything our band does is in a constant state of evolution.