I'm a self taught developer that currently has more than enough experience to hold up against my colleagues waving with their degrees, yet I feel that I'm lacking some important skills to advance further into being a senior level professional in a leading role. More specific in the engineering, planning and designing aspect of software.

I've touched the surface of UML, ERM/ERD, have experienced both waterfall and scrum projectmanagement, ... yet I feel there is something missing as every time I start on a new project I don't know where to begin. Should I start diagramming and how? should I start writing an xx page document describing the project on a technical level first, should I dive head first into writing the first tests and code or pseudo-code?

I would like to know what, in my case, would be the best way forward, to learn how I can tackle this problem in the future and get better at leading and starting a project. There is not much i don't know about my technical tools and languages but when it gets abstract i'm in trouble.