I wanted to add, somethings come easier to some people.
I, like most I'm sure, rarely do all the reading associated with classes. I only read if I have a glaring weakness, usually the lecture material is enough to get strong A's.
I know some people that will do ALL the reading, laboriously and spend tons of time working out of the book. Some of these people aren't very "intelligent", as they have to do tons of repetition and have a lot of exposure before they catch up to speed with the other A students.
Just realize that. When you say you are weak in chemistry someone may ask, do you have a great textbook? If you don't did you go buy a better textbook (older edition is cheaper)? Did you buy the problems book and do the problems?
I heard of this one MD/PhD who went to a tough school, she did EVERY problem out of the back of the book. EVERY one. I thought "wow, I usually only do the assigned stuff." Imagine if your professor assigns 25 problems or 30, and you did 70 problems. Would you be "not very good"?
But the thing is this, in the end if you reach your goal it doesn't matter how you got there.
Are you doing all the reading? Are you doing every problem from the back of the book? I find it hard to believe you would still be not very good if you did all this. Some people do. If you aren't very good and you still want to be good, sometimes you have to put in that sweat equity.