-Taking the non-thinking NPC college route (going to whatever school was close enough to commute to, majoring in classes you thought were easy or letting your family influence your decision too much)
-Getting married extremely young because you think (probably correctly) that you can't support yourself, and then staying married much longer than you should have for the same reasoning (probably incorrect, by then).
-Spending most of college trying (and failing miserably) to take a large course load before realizing that you really just can't do it, and the only way for you to actually succeed is to only take two or three classes at a time so that you can do perfectly in all of them (i.e. stop trying to fight your perfectionism if it's causing you to fail, and just give in to it).
-Waiting much too long to start on a career track.
Upvoted for your mistake #3, which bit me as well (except it looks like you figured it out before graduating, whereas I didn't).
This is a repository for major, life-altering mistakes that you or others have made. Detailed accounts of specific mistakes are welcome, and so are mentions of general classes of mistakes that people often make. If similar repositories already exist (inside or outside of LW), links are greatly appreciated.
The purpose of this repository is to collect information about serious misjudgements and mistakes in order to help people avoid similar mistakes. (I am posting this repository because I'm trying to conduct a premortem on my life and figure out what catastrophic risks may screw me over in the near or far future.)