Will a low grade in my final year of an undergraduate program affect my chances of being accepted into graduate school?
This is March.  Isn't a little late to start applying for fall?

If you get a conditional acceptance somewhere, you should be able to satisfy the conditions with summer study.

For a PhD, grades are not the be-all end-all -- as long as you are not falling out of good standing.

As a PhD student, you'll need to take responsibility for your progress.

You could start preparing for your new role as a student by pretending that you are already a PhD student, and focusing on things that matter more than a letter grade.

Think: what other criteria can help you decide whether to drop a particular course?