Amortization schedule

I have an excel spreadsheet for tracking a self-financed property I am selling. Initially, the spreadsheet was set up to just do standard amortization. Years ago, when I was still doing a lot of Excel in my job (retired in 2016), I modified it to do reverse amortization because sometimes the tenant would miss a payment. Two things happened. Mainly, I've forgotten excel...and secondly, as I've approached the end of the loan term (say 15 yr), I now see that the sheet still thinks it ends on time (12x15 yr or 180 months) but it doesn't because the tenant missed payments and the loan is not paid off on month 180. So the spreadsheet needs to extend until the principle balance is zero. Everything else seems to work ok. But I can't remember enough Excel to modify it such that it will do that. Whether my sheet can be modified, or someone has one that does what I need, I am looking for an accurate tool to track the payments.