You hear it left and right - "hard work pays off"... but there's a missing piece to that statement. How about "hard work pays off... if you're working on the right stuff."
Let's use tests as a very relevant example. Each test will have a percentage breakdown of topics, and there are clues to help you sniff that out (the practice test, the breakdown of lectures, professor's statements). Your first midterm in calculus might be 70% on limits and 30% on derivatives. This breakdown should guide the study time devoted to each topic. In this case, you can accept less confidence on derivatives (30%) in exchange for high confidence on limits (70%). "But derivatives give me nightmares! I need to study these becuase they are new and scary!" Trust me, I get it. But in this 70-30 case, doing a great job on limits and a bad job on derivatives is better than a mediocre job on both. Be rational rather than emotional about the way you spend your time studying.
This comes down to the concept of leverage. Each hour of time spent has a certain leverage, or influence on a desired result. Quite often it can feel like you are being productive because you are working, but those hours spent have little impact on the result, ie low leverage. Studying content which make up a big part of your test has high leverage.
In practice, I recommend that before you embark on any big effort (studying for your calc 1 exam, for example), you devote at least 30 minutes to planning that effort out. That time you invested will be worth it 10 times over. So many people just dive right in, and realize afterward that many hours were wasted on things that didn't end up being important.
Keep it up young penguins.