How do you study for chemistry ii?

Hi everyone, first post here. I had a rough time in gen chem 1 (failed and retook with A-) and now I am having the same trouble in gen chem 2. I study sufficiently, do lots of practice problems and really understand things where I can explain every concept and how to do any problem to a classmate. The problem is when the test comes, I don't know what happens but I make enough stupid mistakes to warrant a C in the exam. When I go back to check the ones I got wrong, it is usually I don't know how to do 1-2 and then ~4 with silly mistakes even though I knew how to solve the problem had I been studying in my room.
I got a 70 on the first two exams and the next one is in a month, it's urgent that I do much better to save my GPA from the sinkhole it is in. Please any advice??
Thank you so much.