How do you cope with realizing how easy an exam problem was to solve right after the fact?

[venting but advice is welcome]

I swear this ALWAYS happens and it makes me feel BETTER to know when something I flunked on was well out of my range at the time.

But I just had a potential make-or-break practical coding assessment yesterday that only had two problems and I felt very prepared because the practice problems were a piece of cake.

I spent almost the whole time trying to throw things together to solve that first problem, while the second problem was very easy and I knew immediately how to solve it but didn't have time to clean anything up so even though technically correct, it was a complete mess and I'll get very little credit as per the rubric. I also didn't get around to the free points from documentation and formatting.

Just a couple hours ago when I was walking home, I SUDDENLY realized how I was supposed to solve that first problem, got home, booted up the IDE, and. IT TOOK 3 MINUTES!!! I actually wanna cry, it was genuinely so easy and I spent the WHOLE EXAM doing that problem incorrectly in a million different ways because I couldn't stop and think.

Side note, I despise yet understand the notion of professors using these kinds of lab exams to flag students with a performance dissonance between their assignments and the exam. I most definitely failed this exam, but I did well on most assignments because I HAD TIME TO THINK! Also, a quick google search for a tiny thing I forgot would have saved me 10 minutes and 20 ish lines of the most sluggish code I've ever written to step around that one thing I forgot.