Kris, don't beat yourself up (too much)

Learn lessons from your own mistakes.
Read up other peoples problems and hopefully learn from their mistakes and experience.

Diagnosis by its very nature is a mix of testing, thinking, more testing, checking your results. Often it seems to take a long time to find what in hindsight is an obvious fault. But if it was obvious it wouldn't need diagnostics.

Experience is something that you can't get with shortcuts.