Relax. Your mental health will suffer from anxiety if you obsess about grades. As above, the classes can get more difficult but they also prepare you to do well in college. You are transitioning from the easy phase of HS to one where best study habits are needed to succeed. You should look at your study habits and form new ones to manage having more difficult material. This will serve you well in learning the material and learning how to study in college. In the past I’ll bet you alreaady had known more of the material to be learned. Now you need to learn more new material/skills and can’t get by with previous knowledge/skills.
So spend more time understanding the material. You are just as smart as you used to be but (finally) encountering material you need to learn instead of already having a handle on the majority. We often don’t realize how much we build on past experiences. Now the experiences are being added to more than in the past.
btw- repetition is how we retain material- I did that above I realize.