Give your projects finite terms. Give them a deadline to have reached a certain milestone by. Having a finite term in which to do so means there is less chance of the project dragging on, and it helps you for future projects as well.

This helps you in setting goals, and it also gives you the chance to refresh, to rethink things through a different lens when it comes time to start something new. Not only will you have learned skills from the previous project, you would have learned what sort of approach to take to new projects as well.

Generally speaking, you want to go in head first. You want to be able to give the project your all so that you have the best chance of achieving your goals within the determined time frame. Depending on how well that goes you can adjust your goals for future projects as well.