More言, take it easy.
微积分 is not rocket science.
Anyone can crack it if he/she really wants to make it.

Here is advice for you,
Preview the textbook and redo the examples until you are really familiar with the content.
Ask your professor something confuses you.
Do your homework independently, and practice until you are fully confident about yourself.

In summary, my IQ is pretty low, with upper method, I made it.
I believe you can too.
Trust yourself and never give up and give in.
Thanks.