Is there anything as bad as writers block? Yes, writer ADD. You know you have so many great story ideas and characters and you’re not sure where you should start. Or you start writing something and then you become infatuated with another thing at start writing it instead.
Full confession I have numerous stories in different stages of planning, writing and editing. And I freely jump between them as my mood strikes—to a point. Right now I have two main concerns: the companion book to Shameless and The Chrysalis Project. Ideally I write the later once a week and the former every other chance I get. I also have those other ideas. And I have to keep those ideas percolating until it is their turn.
If discipline were enough then everyone would be writing all the time and Game of Thrones wouldn’t be finishing before A Song of Ice and Fire. (I love you George!) No, routine and structure allow you to focus on one (or two) thing(s) at a time. It’s fine to once in a while write something else if you hit a rut in your main story, it could be useful. But it can also be fatally distracting.
I have something that keeps me focussed and that’s a schedule. Nothing like deadlines to keep you focused.