I've been working on a project for 3 years straight. The hardest part of continuing the project is that 90% of the project gets done in the first year, then the 10% takes so much time it's very discouraging.

For me, it's the feeling of accomplishment and pride that keeps me going. Regardless of the projects success, I'm proud that I've worked on something for so long and seeing it improve everyday is satisfying. There's nothing really else out there that gives me the feeling. If you ignore the success of the project (whether that's monetization or popularity.) and focus on the fun and beauty of improving something every day, it lets you go pretty far and long.

At the end of the day, you always have that project that you worked on and improved on every day. You can look back at it, show it to people, etc, and it almost seems bigger than you.