I have the EXACT same problem. I am pretty sure it’s at least a low form of ADD/ADHD and there isn’t much to do about it if it is. I think people like us need an external source of steadyness, though this isn’t always possible.
Creating calendars or schedules for yourself (even if only for one project; June 1, film. June 2, edit…) and MAKING yourself stick to them I think is the only thing you can do about it.
I suppose the best advice I can give is figure out some way to only work on one thing at a time. If I had a studio and a crew, this wouldn’t be hard. Being you and I are just people, though, it has to be internal. Don’t cancel stuff for this reason.