You have to be that manager yourself — breaking the app or product down into small components and working out what should get done first.
Build a list in a form you’re familiar with — even if it’s a form you don’t like such as Jira. Break it down into large chunks and smaller assignments. Then start on the small stuff, ticking them off on by one.
When you see tasks being completed and the list moving, it builds a sense of momentum and it gets easier to keep going day after day.
If you don't know what you're going to work on or haven't decided what to build yet, then stop what you're doing and work this out first.