First of all, I don't think you should just tell yourself that you are not "good at math" this early on. When I was in high school, I thought math was just not my thing. There are so many different ways problems can show up in an exam and I just wasn't able to answer. Formulas don't come up when I got nervous... all that stuff was bygone. When I went to college, I thought I wanted to do Economics, but since I still had to take a couple math classes to fulfill the major requirements. The simple motivation of fulfilling a major requirement turned into something. In one of my classes on probability, my professor was so good at teaching the concepts that I fell in love with the class, and I ended up taking an advanced probability class the semester after.
What was more encouraging is that after my junior year, when I went to career fairs, insurance companies and accounting / consulting firms saw my resume and encouraged me to take an actuarial exam. Surprisingly enough, I passed my first exam, and I ended up getting a job in the field of pension actuary. Every time when I pass another exam, I felt much more encouraged and confident that I can be good at math!
So the bottom line of my story is that, don't ever believe that you are not good at something. If you put in efforts, and with the right mechanism of encouragements and rewards, you well be good at it. So don't let your belief that you are not good at Math stop you from major in STEM!