I think that you might have taken the class when you were too young. You really will get stronger as you get older.
Is there any way that you can take a W, and then retake the class as a senior?
As you progress in math, physics will get easier.
The other option is to seek out more help from your teacher, or to get a tutor to help you in the class.
In the future, be aware that there really are classes that you are not ready for, but that you will be ready for in a year or two. A lot of things in math and physics depend upon things that you have not learned yet.