I second that. Without taking notes/diagramming no mere mortal can handle nested 9 levels just in the brain. And somehow I doubt that the OP colleagues are doin

I second that. Without taking notes/diagramming no mere mortal can handle nested 9 levels just in the brain. And somehow I doubt that the OP colleagues are doing just that either. The diff between people being stuck and progressing is often how much and how smart they writings things down. My suggestion would be to try to tackle the low hanging fruits first: find the parts of the lowest complexity, try to understand these and get the simplest things done first. This will also restore your confidence.