I'm 22, and perhaps I'm naive in my belief of this, but one of the biggest things I've learned is that theory and practice are rarely aligned. Reading about mistakes someone else has made generally only helps you see that you've made the same mistake in hindsight; when you're heading down that same path, you tend to justify it or are completely blind to the fact that it's a mistake.
All that said, I'm curious about what comes of this thread regardless of how useful it ends up being.
You need to constantly validate your theories against the real world, what other people say is not enough.
This means: solve a lot of problems after you learned a theory, try to feel out the problem space yourself by extending the theory or applying it in a specific way, practice, practice, practice, and always look out for things everyone considers true but are not obvious from the data.
I am finding more and more as a 21 year that all the answers that you need come from within. I can read the internet all day long for advice about my life but at the end of the day I am really starting to think I am the only one that knows exactly what I should do. Actually doing what I know I should is a completely different thing however.
All that said, I'm curious about what comes of this thread regardless of how useful it ends up being.