I feel for you. I did pretty bad on N2 too, especially listening since I don't have time to practice it. But I need to throw those feelings over and work on. Anyway, good luck and I hope we both pass. XD
Btw, the idea of studying for N3 isn't bad, since N3 contains the basic of N4. Heck, even in the grammar section of N2 yesterday I found plenty of N4-N3 questions, very few of N2. You can go back revise your N4 knowledge anytime when you studying for N3 as most of it are the key to advance to higher level.