Jennifer, popular practice is to go into the test consistently scoring at least 80%. If you have not met that, then I do think you need time to study.
To better assess your own confidence, ask yourself how long you have been studying (months, weeks)? How many tests have you taken (1-5, 6-10). What websites and programs did you use to practice test? List the number of knowledge areas you feel you are weak in.
If your answers inspire your full confidence in the knowledge, then assess your test taking skills. How many full length 4 hour exams did you take? How many breaks did you need? Did you have time to review within the test time? Maybe it's a matter of planning how to take the test.
You do need to make a decision quickly, though, because I think if you wait too long to change the test day appointment, you lose 100% of your fee.
I am scheduled to take the PMP exam in about a week. However, My scores on the pmp simulator practice exams are not in the 80%. My last score was 74% and I plan on taking another exam this week to see where I stand. Do you think it's a good idea I postpone my exam until I start scoring in the 80% consistently with the PMP exam? Or take the exam and see how I do?