A deadline set by a boss or teacher is much easier to make, than one you set for yourself.
An outside push with consequences if it's not met.
One you set for yourself doesn't have those consequences. There are ways to make it hurt if you fail to meet them, like paying a buddy if you don't make it.
Creating a self-imposed deadline for a task can kick one into motion, but there are some things to note to have a better chance at success.
Don't set it too far off into the future. Instead of years or months. Try weeks and days.
The closer the deadline, the more sustained action you have to take to reach it. Urgency can get you to just start instead of making excuses.
Even if you think the project is too big, break it into smaller parts. As a whole it might take months, even years, but tackle smaller pieces with more reasonable deadlines.