At my school, we were in the same AP class the entire year as opposed to one semester. Thus, college classes tend to go a lot faster. In high school we covered a chapter a week with classes meeting every day or every other day. When I was in college, I covered a chapter per class.
Additionally, not to put down any high school teachers, but the college professors had a lot more knowledge on the subject matter. My AP teachers in high school tended to only have a B.A and very seldom had a masters in education. When you go to college, you are typically learning from professors with doctorate level education who has studied this for years and years.
Honestly, there is such a huge distinction between AP and college classes and it can go both ways. Sometimes, college classes were even easier for me than their AP counterpart. However, I'm sure college chem was harder than my AP chem class in high school. Every place is different, high school and college alike. I'd say the biggest predictor in how well you will do in college is your enjoyment of the subject matter. If you truly enjoy and are excited by what you study, there is no way you will do poorly. If you do not like the particular class you are taking, try as hard as you can, go to office hours, and show the professor that you are putting forth an honest effort. If you go to a big university, there is always the worry that you will just become another face in the crowd. Even if you are shy, try to speak up in class, sit in the front, and GO to office hours. This can even open up opportunities for outside the classroom like shadowing, helping with research, and even teaching assistant positions (at smaller schools)