You do not have to sign it. You must reaffirm a debt while you are in bankruptcy. Moreover, there's no reason for you to reaffirm -- if you remain current with your payments, the bank will not foreclose.
The law does not require reaffirming mortgages to keep real estate. No attorney that I know around the country would sign a reaffirmation agreement on a mortgage, at least not without concessions. Regardless, if it is not done prior to the discharge, it is not valid.
[This communication is intended as general information and not specific legal advice, and this communication does not create an attorney-client relationship.]