The "lady at the travel agency" should have never given you that advice, because she's wrong.
It could be a big problem. It's really going to depend on how diligent a check in agent you run into at embarkation. The rule states you must have a notarized letter to travel with one parent or guardian. Some of the people working the check in may not be so thorough, and you could be fine. However, it is possible for them to deny you the right to board.
Perhaps, had the travel agent given you the proper information, you may have had time for your mother to fed ex a notarized letter. I don't know if you still have time before your sailing to get that done. If you do, that would be the route to take.