If your marriage is real you don't have to worry.
Think of it this way :
INS had something against you when you came for your first interview ,and they were determined to deny your application right away, that's why they asked your wife to call off her petition. But it didn't work out (for them ), they failed (not you ) so they had nothing to do but ask you to come for a second interview to get more proof. It's ,of cause, not pleasant but still there is hope, and you shouldn't give up. Get yourself together, get better prepared, have a good lawyer with you, try to look calm and sure of yourself and your marriage. I also heard ( I don't know if it's true or not) that in the waiting areas INS may have hidden cameras to see how couples behave themselves before the interview. So try to be relaxed ( though it's not easy) , give your wife a hug or a kiss while waiting and prove them wrong at the interview.
Best of luck.