This is a very common sentiment that I see in my students (I am a teaching assistant in a university sometimes). I obviously can't diagnose you, but I do recommend speaking to a counselor or a therapist. What you're describing sounds like either burnout or depression, and without more information, I can't really guess as to which one it is.

As for practical academic advice, I have several guesses as to what's going on. My first guess is that you have testing anxiety. It could be worth doing an online search about how to manage testing anxiety and speaking to a counselor if that's the case.

The second guess, which is more cynical but unfortunately significantly more common in students, is that you might be experiencing the big-fish effect. As for the big-fish effect, college is different from high school. There's more people per class, and more people in general. And college tends to have a weeding effect, where people who are less academically inclined tend not to attend college. So whereas previously you found yourself to be academically gifted among your peers, now you find yourself just as gifted as everyone else around you. Big fish in a small pond, essentially.

I can't say for sure if that is the case for you, but it's important in general to remember that college and onward are significantly different from high school and before. This culture shift is the one thing that really trips up a lot of students. In high school, teachers' jobs were to funnel students to the next grade, even if students didn't quite understand the material. College is really the first step where students start to be weeded out. What that means is, even getting through college means you are academically gifted. But it also means that you cannot use the habits that you've used in high school and just expect it to work in college. All instructors in college and onward approach class with the mindset of "you're responsible for your own success." Essentially, you should learn to be extremely proactive in your learning experience. You might have noticed that class takes up much less time than in high school. That's not because professors wants you to have more free time. It's because professors expect you to use that time to stay on top of all the lecture material on your own. Go to office hours. Ask questions. Read the textbook, take notes, study your notes. And if you aren't already doing this: ATTEND ALL THE LECTURES. NO EXCEPTIONS.

If you are having trouble with the material or if you notice that you're doing worse than you expected, then college expects that you need to take the reins and reach out for help yourself. Professors are excellent for answering anything you're confused about - go to their office hours or send them an email. TA's are also a remarkably good resource, but for slightly different reasons: they've taken the exact same class only a handful of years ago, and they know exactly the little tips and tricks that'll help you succeed in class. And they know what's the important information that you need to know and what's unimportant extra information. And they also know what the professor is like, so they may warn you of trick questions or misleading wordings.

As for what you can do right now regarding your grades this semester? Like I said - be proactive. Reach out to the professors of those classes. Ask for clarifications on any questions you have, ask for corrections on any questions you missed on exams. Ask for extra credit. Go through the end-of-chapter questions and do them all, and show it to the professor. Show that you're willing to do extra work, and then actually follow through with doing that extra work. There's an open secret in college, which is that professors can do whatever they want. Convince them that you care, convince them that you deserve extra credit, and they will give you extra credit.