I agree that you should ask for more time if you feel you need it. However, even if your manager refuses, I would encourage you to find one or two experienced nurses on the unit to mentor you for the next 6 months or so. Could be the nurses who precepted you before, could be someone else--doesn't matter, as long as they like to teach and are supportive. Go to them with specific questions during your shift and ask for help in prioritizing/organizing your care when you start feeling overwhelmed.
I also agree with 3rdShift that feeling overwhelmed is not an unusual state of mind for a new grad. However, sometimes, through oversight or sheer necessity, new grads get assignments that are really not safe or apppropriate for them. I definitely saw this as an ICU preceptor, and had to intervene with the charge nurse to get the assignments changed. If you feel this might be the case, ever, talk to your preceptor and/or charge nurse as early in the shift as possible.
It'll get more comfortable soon, I promise.