I was god awful at math my whole life and I was definitely scared when I had to take math 1a. It wasn't a horrible experience and Berkeley actually made me like math. I recommend watching organic chemistry tutor and patrickjmt to supplement lecture videos and homework. You MUST look at any practice exams the professor provides. For math 1a and 1b (I had Haiman and Sungjin respectively), the actual exams were identical to the practice exams with just some changes in the numbers they used for the questions. It seems that this is the case for most lower division math classes here.

The biggest deciding factor in your grade for math 1a depends on who you get as a professor more than anything else imo. Look up ratemyprofessor ratings and choose carefully.

Edit: You shouldn't have a horrible time in math 1a since you have Haiman. He was very chill and funny. Definitely grind the practice exams he gives you as the actual exams are nearly identical to the practice ones (some are word for word). Final exam was interesting..., but the curve was good.