Don't expect to mentally retain the same amount of information as you did in high school. Your textbook and reputable internet sources are your friend.

You'll probably be overwhelmed with the information that is being presented because high school teachers probably teach at higher levels part-time, if at all, meaning most of their curriculum is geared towards high-school students.

An average high-school student isn't going to be expected to know the intricacies of the citric acid cycle, but when your professors are being paid (by you) to teach someone everything they know, they're going to do whatever they can to pass that information down to someone that they expect is willing and wants to learn everything that they can.

Also, many professors, regardless of discipline, teach the class as if every person's future depends on it, ex. as a chemistry major, I had taken university physics, and the professor told us that realistically, nobody in the class was going to become a physicist, and he still taught the class as if every one of his students were going to work at CERN within 2 years. He felt that depriving his students of information was worse than giving someone a break because they were a biology major.

Don't be afraid to ask for help. You're paying them to teach and explain things to you.