I'm so glad to learn that I am not the only one to have debacles of this sort. I failed the midterm exam in Queueing Theory while studying for my master's degree at Stanford. I eventually squeaked by with a C- grade for the course by studying incredibly hard, including twice-weekly visits to the TA. I got bad grades in several other classes too. Anyone who sees my academic transcript always says, "Wow, I never heard of anyone getting such low grades in graduate school!"

I like how you concluded that paragraph: "...and here I am, haven't died because of studying for an exam :)"

Ironically for me, 4 years after that C- grade, I accepted a storage products performance job at IBM. Most of the day to day work required understanding and original formulation of queueing models. Despite my horrific grade, I had learned the basic theory and applications of queueing very well! This was due to my fear of failing the course; it was required for graduation.

Hmmm, come to think of it, just a few days ago, my much younger friend on Twitter asked me how to model discrete processes involving random arrival times and Poisson distributed service times. He is a college senior, applying to PhD programs in computational linguistics, and very very bright. I felt so happy to link him to one of my answers on StackOverflow about M/M/1 queues and Kendall notation!

What you wrote here is lovely. It is one of the most authentic things I have read in awhile. I wish you the best of luck in your future endeavors. I suspect that your father is more proud of you than you can even imagine.

P.S. I have another Twitter friend, who is probably at a similar stage as you in his career. A few years ago, he quit his mathematics PhD program at a prestigious American university and returned home to Portugal. Until recently, he had an excellent non-academic job using his fine quantitative abilities. I think he will feel encouraged by reading your post so I will share it with him. Your efforts here are appreciated.