Regarding the work on the app, perhaps you could detail what you will do before you begin. That would set boundaries, so that the task does not expand as time goes on and they think of new things to add. I understand what you say, people sometimes are not aware of how much work goes into delivering such things.

From the way you analyse your behaviour and how it impacts others you really don't sound like a selfish person to me . In any case I think there is a difference between being true to yourself and being selfish.