Paul Graham, writing about How to Do Great Work:
Since there are two senses of starting work — per day and per project — there are also two forms of procrastination. Per-project procrastination is far the more dangerous. You put off starting that ambitious project from year to year because the time isn’t quite right. When you’re procrastinating in units of years, you can get a lot not done.
One reason per-project procrastination is so dangerous is that it usually camouflages itself as work. You’re not just sitting around doing nothing; you’re working industriously on something else. So per-project procrastination doesn’t set off the alarms that per-day procrastination does. You’re too busy to notice it.
The way to beat it is to stop occasionally and ask yourself: Am I working on what I most want to work on?" When you’re young it’s ok if the answer is sometimes no, but this gets increasingly dangerous as you get older.