ok so i know it sounds like a really stupid question and that i should just upsolve and learn from my mistakes..
but how? how do i learn from absolutely idiotic mistakes from problems that i should have definitely been able to get through, and how do i stop this from happening in contests again?
also, how do i deal with problems that i have no idea on how to even approach? like for C, i've done a bunch of standard tree stuff on usaco, but i had no idea what to do after figuring out that an edge is useless if it doesn't cross the 1->n path, and how to calculate whether to keep the edge or not when it does.
honestly just a bit of a rant i'm kinda salty i gave up on B after a silly mistake and then proceeded to fail at C miserably for an hour...
A lot of it is innate, but. you can draw observations for problems you dont know how to approach. If you have spent 10 minutes, and have no observation of meaning, please skip to the next problem. I did this once, and as a result solved my first 1900 because of an impossible 1400. If I didnt skip that 1400, I would have performed very poorly.
As for stupid mistakes, yeah those happen and you have to accept it. Rather than worry about these variances, skip a problem if something happens, so you come back to it later. Often this will let you solve the next problem (sometimes 1800-1900) and that alone can save your perf. Timing matters too though so solving larger weight problem early still gives advantage of 300-400 points often. If you still have time, then you can come back to the problem you struggled on as you see fit.
I generally don't like jumping around unless the standings obviously confirm that the next problem is easier. I find that the moment I start doing 2 problems at once is the moment I am guaranteed to fail both because I end up not trying hard enough on either and just switching every time I get stuck.