Ask HN: How do you stay motivated to pursue a project / problem for a long time?

I have noticed that, over the last few years, I have not stuck with any project beyond 8 months at a stretch.

I am very impressed by people who stick with a problem / project / startup they are pursuing for a long time (> 5 years).

How do you maintain motivation to keep pursuing a problem / project / startup when the going gets tough ?