With any luck you would have caught the discrepancy, but in case you didn't, having the name of the person who gave the faulty fact will protect you from being blamed. Or at least you can share the blame with the person who first wrote it. He or she wants you to write about your ideas, not someone else's. Regardless of what grade you get, if you've done the work yourself, labored through researching, suffered through the writing process, toiled through revisions, and produced your own work, you can be proud. Having something of your own, an original work in which the main ideas are yours, should be reward enough.
Grades are important, no doubt.
But ultimately, being able to produce your own work, to take pride in it, and to have a sense of personal accomplishment are more important and cannot be found in plagiarism.