I think if I were able to give a single piece of advice to my 18-year-old self, it would be that.
If I were to follow my natural instincts, the amount of effort I put into a situation depends on how happy I am with it. There is a formula for that:
happiness = results - expectations
I've been most happy in situations when I came from worse environments. I loved Caltech wholeheartedly, partly because I felt like a fish out of water in high school in Las Vegas, and Caltech was such a welcome change. If I'd gone from an amazing high school, I probably would not love it as much.
Thus, it doesn't make sense to base your effort on happiness then, because you'd only end up putting in a lot of effort if you had a bad experience leading up to it which forced you to lower your expectations.
Once you decide to do something, even if you have a lot of doubts, you should just give it your all.