Sideprojects follow the same rule as with every other project: they can be daunting. It might be overwhelming to get started. Or you tried to start and the number of unknowns grow on you. Finishing something is even a complete different story: you might need to wrestle trough difficult steps or complete a set of boring tasks.
One thing that always helps me to get a project moving (again) is to focus on the very next action I can do. Planning your dream vacation might be a huge project, finding a suitable window in your calendar should be something you can sit down and finish. Use this momentum to get to the next action, and the next, and the next.