You get much of your knowledge by doing mistakes. Here are some things I would have done differently if I had my present knowledge:
1. Choose another second foreign language
In school we were able to decide between Latin and French as a second foreign language (besides English). I decided for Latin, because everyone told me, French grammar is more difficult and Latin is more logical (I preferred logical things, because I also liked maths). In the first years I didn't regret it, because my grades were great, but since that changed, I do a little.
Latin is pretty useless for me and it's also no requirement for my study. With French I would be able to talk to French people in their language or I would be able to understand them. Now I can only guess some words in other Roman languages, because they have some similarities with Latin.
2. Taking a year abroad
I don't know why, but especially near the end of school, one of my main goals was to get school done as fast as possible. I was excited about going to university. I didn't really think about taking one year abroad, like many other people I know, do or did. One year traveling through another country like New Zealand, getting to know new cultures and improving my language skills. I was too blind to consider this option.
One reason may be, that a lot of things in my life went wrong at that time, where I would have to decide about it. It's a lost opportunity, but at least I took one month to travel through Europe by train. It was a great experience, and I really miss it.
3. Staying in my home town for studying
During the final year of school, I applied at around 3 companies for a "dual study" (study and apprenticeship) and one company agreed (I'm really thankful for that!).
But because I wanted to stay in my comfort zone, I didn't want to leave the familiar environment, I didn't want to leave the flat I was living in for 18 years, I didn't really think about applying at a company somewhere else, like in another city. Another lost opportunity, but nobody holds me back from moving out of town in the future.
You see, almost everything is connected to my fear of leaving my comfort zone. I was too lazy and wanted to go the easy way. I wanted to keep the effort at a low scale.
Sometimes I was just too stupid. But now I'm wiser and I'm here to advice people doing not the same mistakes, I did.