I'm not an expert, not a person with ADHD, nor an accomplished developer who manages to finish his projects constantly. I have a few ones going on for years and I struggle with the same issue both at work and outside of it. BUT lately things have improved, and by a very simple method: I keep a journal. Specifically, it's to record what I do daily, what I read, the projects I start, etc. Then, I keep a list of daily, monthly and yearly objectives, and at the beginning of each month I plan my goals, which are mostly projects I want to complete. I keep a list of open projects, sort them by priority and stuff them in there each month. It's not automatic thet I manage to work or finish the projects, but at least it builds some pressure which leads me to remember about them, so I'm compelled to work on them. This month I managed to finish my new website revision, which was an outstanding issue since last year's june. This makes me happy, it's a push to continue on this path, and it lift my morale as I see I can complete things.