This used to be an issue I struggled with. I found sprinting towards small, but functional deadlines was effective. This way even if I put down a project for a few months and then came back, it was in an ok place to pick back up again and sprint some more on.

I also found that being able to complete one small task and then switch gears on the project kept me more entertained.

Also working with a good coding partner can be helpful, but that takes it's own time to setup and organize.