I understand your situation very well. My favorite course was Biology in high school. I was fascinated during the teacher was speaking; I was very interested about the subjects and read and investigate more at home. On the contrary, at the exams I get the worst grades from Biology where my other grades were very good (except history:) ).
To be interested in a subject is not same with memorize it with the all details. Yes, we are smart people; we understand everything we read at once. But, when it comes to the exams, we get bored and stressed memorizing all the details and old Latin names. In addition, even if we do not realize, our subconscious do not appreciate the value of memorizing, we are only read for fun and learn. So, it is not that you are not good at reading or other things.
It is same for the drawing, if you have a talent to draw a straight line, you can always be good at drawing by working hard. I suppose you are not that passionate about drawing like me.
I also like drawing when I'm bored during long meetings :) But, I have never dedicated myself to drawing. It would be always nice to use your skills in your daily job.
When it comes to Math, you will never ever found a major that is easy to complete :) You are a smart young girl who can analyze herself and take the right decisions. With hard working I am sure that you can finish your courses. When I was at college, I had study groups for every course. Even I cannot understand a subject; there were one friend who can overcome it. If we all did not understand it, with the connections of the group members, we found someone who understood it or we go to the lecturers to ask them to explain us again.
Let me tell you about the working life. When you become a civil engineer, you will not deal with any math 4 operations. All the designs are done by the special programs and tools but you will understand how it is calculated and which variations are important during the civil design.
I would be glad if you keep me updated about you and you can always ask about anything.