Listening to the school counselors who claimed that I was incapable of learning a foreign language. Three districts later, the counselors started urging me to take a foreign language. Signed up for Spanish I in my junior year of high school and Spanish II as a senior; learned enough to get by in conversation with native speakers.
Forgot most of it over the past 25 years due to being out of practice.
My biggest mistake was trying to take Calculus as a Freshman and Physical Chemistry as a senior. PhysChem seemed like nothing but Calculus that I hadn’t even thought about for three years.
This was right at the end of my aspirations as a biochemist anyway, but for a few weeks, I struggle to re-learn calculus while trying to keep up with the main class. With two lab sections (literally 30 hrs/week in classrooms) and a part-time job, it was just plain impossible to do it all and I dropped out of the course about halfway through.
Of course, one could say that pursuing chemistry itself was the biggest mistake. Once I realized what the accounting profession was like (a long story there), it was so obvious that it was the perfect fit for me. The problem is that the high-school me was given a skewed view of some career alternatives by well-meaning teachers who nevertheless seemed to want to vicariously fulfill their own personal ambitions rather than take into account what I should really be doing to fulfill my own goals.
I’ll definitely echo this. I feel I finally ended up ok now, but I guess I probably got set behind in life/career at least 5 years, maybe even 10 years.
Within this, during my spell in college I wish that I had done internships, apprenticeships, etc. Guidance advisors at my university were beyond clueless about the realities of post-graduation job searching and hiring.