I went through the same fears when I started (boy that seems like ages ago!

). I had a lot of problems w/ math and science in high school, and bought into the idea that I "just don't do well in those areas". So when I got to college, I took a math course that wasn't too difficult. On my first exam, I got a 56%. That just broke my overachieving heart! I started to go see the TA 3 times a week, and really started disciplining(sp?) myself.
Once I started to change my attitude about math and science, (i.e. instead of believing that those were areas I simply couldn't excel in, I started saying "I can do
all things, including math, through Christ". It doesn't have to be that exact phrase, but something that encourages you), It just got easier to understand. On my next exam I got a 98% (HALLELUJAH!).
The point is, a lot of times we buy into ideas or concepts that aren't necessarily true. The more you believe you aren't good at a thing, the more true that will be. And I guarantee you, no matter when you take those classes, it will still be just as hard until you decide that you
will succeed, no matter what.
If you are still unsure as to when you should take the classes, look at the rest of your schedule. If you have an abundance of classes that will challenge you intellectually in other areas, you may not have the time to devote to math/science. But if you have 9 credits of something you are really good at, then maybe you can take 3 credits of math as you'll have the time to give it. God bless you, and I know you'll make the right decision!
But no matter what,
ENJOY you undergrad years to the
fullest!!!