I strongly echo the other replies to your quiery. The CEN is about experience - NOT booksmarts. Do your time, see all you can, soak it all in, take ACLS, PALS, NALS, ENPC, TNCC....then worry about your CEN a few years down the road. As a green nurse (no offense, I was there once too) , the saying goes, "you don't know what you don't know yet." The best thing you can do for yourself is get a great preceptor/mentor and an excellent orientation, stick your nose into as much as you can, stay curious and motivated, and don't rush yourself to make some unattainable "time-table" -- it's a set-up for early burn-out.
Most of all, enjoy your experiences in the ER. I wish you well in your (upcoming) career choice.