I wasted a lot of time during my PhD. As a postdoc, I don't see much progress in my abilities. How can I make progress?
My sense is you're putting a lot of weight in past mistakes, and you got your PhD in the end. The rest about your bad understanding of the material or weak research is hard to evaluate objectively, what does your advisor say? Other mentors in the field? Getting a more realistic picture might help you be productive moving forward.
the concerning thing is no job offers, but your advisor bought you some time. I'd keep trying for a postdoc (maybe apply more widely if research is what you really want) and if you don't get anything again, then start shifting into another career track.