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? I made several mistakes in my PhD:

Joined a PhD advisor who didn't have direct knowledge/experience about my PhD topic. She worked with nanomaterials, my work was on continuum mechanics. I had both experimental and computational experience during my master's degree, but my PhD thesis was completely computational research.
During the first semester, I was not focused in my work and classes. I thought I was in the wrong place.
Feeling pressure from my colleagues who published 6 papers during their PhD, I was more focused on getting published, rather than getting knowledge. I did not take more courses as necessary. My research turned out to be less rigorous.
I got overwhelmed with my low output during by 3rd year, as a result I lost almost 4-5 months as completely unproductive.
Even though I was no longer foreseeing any advantages of doing this mediocre PhD, I did not want to leave my PhD halfway. The more time it took, the more I lost confidence in my abilities. The available jobs in my field required high level of expertise, and I had very limited experience.
However, I got a PhD earlier this year after 5.5 years. And unsurprisingly, I did not get any job or postdoc position. Also covid struck, so that didn't help either. So, my PhD advisor offered me a postdoc position for a year and I had to take it as I didn't have anything else.
Now, for the past 4 months as postdoc, I have made some progress on my knowledge, publications and research experience. But still I don't think I can get a job in my field.

I think I am in a loop. I don't know how to progress. Based on the facts above, do you have any suggestions/advice on how can I improve my career?
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.