I hope I am not at the end of the road. I had flunked advanced engineering mathematics and applied mechanics during my undergrad second year. Though I scored decently in the reexamination during the subsequent semester, I never recovered from the blow to the self-esteem from flunking first and last time in my education.
I liked applied mathematics and mechanics, though I was never good at it. I was very good at experimental mechanics but I hated the manual process of sample preparation, lack of theoretical studies. When it came to choose a research field during master's degree, I chose computational mechanics. I figured, though I might not be good at this, I can always learn and not repeat my undergrad debacle.
My error was, not taking enough courses. My grad school courses were not mathematically rigorous enough and were quite superficial. I did not take the harder courses as I feared failure and repeat of my undergrad flunking.
My PhD advisor, committee did not force me to take the rigorous and advanced courses as I had a PhD topic that did not involve much rigorous mathematical framework development. I did not ask help and guidance from professors due to my social ineptitude, worrying about what they would think about me. I should have taken the 3 graduate level courses when I had the opportunity. I believe, I would have had more confidence, had done better PhD research.
Now, I am doing postdoc in same topic as my PhD. I am searching for postdoc positions and industry Jobs, I find requirement for candidates with rigorous research experience and graduate level coursework. I have none. I have published papers in non-rigorous journals. I am still afraid of mathematics and numerical methods.
I have screwed up my career, now I would like some advice how I can recover? I should have done things differently 5 years ago. I am old now, and inept at my field of study. I have been trying to self study for the past few years, I still suck big time. I should have stayed in experimental mechanics research. I was natural in it. Computational mechanics is not my cup of tea. I bit off more than I can chew.