Take this with a heavy grain of salt. It's an idea I've had -- even though I don't have motivation problems -- that I have yet to do. (I plan to within the next three months.)

Hire a part-time project manager.

A good project manager is immensely helpful in (1) teasing out and decoupling requirements, (2) producing a work-breakdown structure (WBS), (3) setting a schedule/timeline for execution, (4) assessing risks, and finally (5) controlling activity and adherence to the schedule.

These are all activities that suck to do alone. A project manager offers a useful organizer and controller. Basically, they represent a forcing function.

Just like personal trainers help unmotivated people stay in-shape, I think a personal part-time project manager would help you follow through on your projects.