I too struggle with ADHD tendencies where I'll suddenly and cold-turkey lose interest in a project I've invested LOTS of time in, and I've found that organization is actually the key to getting me to the finish line! I do everything on Google Drive, because I can access it anywhere (phone, iPad, laptop, etc) and it's easy to organize writing docs/spreadsheets into folders. AND it's free.
When I get a plot bunny/new project idea, I go into GDrive and make a new folder for it. Then I just dive in -- outlines, character inspiration, rap sheets, chapter writing -- until I inevitably lose interest. When that happens, I can go back to GDrive, look into all of the OTHER folders I have, and dive into something else that interests me. It's kind of a round-robin process.
Working this way helps keep me from stressing, too. Sometimes I would feel so guilty about disliking a project where I've worked so hard, that I would have to completely delete it out of my life to prevent a kind of shame-guilt complex. AND ALSO having all of the projects available to me to pick and choose from saves me from ruining a project just because I feel stubbornly obligated to keep working on it, even when I'm completely over it, haha.