How do I reaffirm my mortgage?

You might be able to re-affirm your mortgage under these circumstances; however 2 years is about the maximum for something like this.  In order to do so you will have to make application to the bankruptcy court (the one from which your discharge was issued) to re-open your case.  Though you should be aware that this is done solely at the court's discretion so it may not be allowed.  If it is, then you can re-affirm your mortgage (and then have your case once again re-closed).

The reason for going through all of this is that by not re-affirming the mortgage in the original proceedings, you were discharged for any financial obligation under it.  Thus legally, while you are still the owner of the property, you bear no personal liability for the mortgage payments.  While your lender still retains the right to foreclose for non-payment, it cannot come after you for any future deficiency in the event it has to foreclose and the sale price is less than the remaining balance on the mortgage.

The problem here arises because you can't legally re-affirm a debt that you are not responsible for. The only way to remedy this is by re-opening your bankruptcy and having your discharge, at least as to this one debt, re-instated. By re-affirming your debt, you are once again undertaking personal liability in the event of non-payment of your mortgage.