Our brains are leaky, cheap, plastic buckets! Sucks, I know. Mine is worse than most, but focused reflection, "debriefing", has been working wonders for me. I wrote this as an exercise for students finishing a semester, but it applies to any life experience. We spend an immense amount of time and energy on things, but in our rush to get to the next, we don't let our learnings sink in. At the end of an experience, invest a bit of time in summarizing the key things you learned, what went well, what didn't, and how you'd improve next time. Write the stuff down, "explain" the concepts to an imaginary audience. You'll have new insights and you'll remember things much better.
An example is reading books (or textbooks, hint hint). I used to read a book and eagerly jump into the next. Info in, never use it, info gone. Was the reading still good for me? For sure, but far less so than it could've been. These days, I underline passages and take margin notes, and a couple weeks after finishing the book, I'll copy those quotes into my own doc. I'll paraphrase each passage, drawing what connections I can to real life ("active learning" like this helps with retention). I legitimately think this 2-3 hours extra doubles the gains I get from a book.
This practice can, and should, be applied everywhere. Do it after traveling to Thailand, after finishing your DIY bookshelf, after your summer internship or leaving your job. After a breakup. Whatever. Spend a bit of time to reflect and you'll see immediate payoff - the experience is wrapped up nicely in your head, you grasp new and interesting takeaways, and perhaps most importantly, you'll do better on similar experiences in the future.