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You can go by the payments you actually made in 2015, whether they were applied to 2014 or 2015. The interest portion of the payments is probably not all that different anyway. You don't need a 1098 in order to deduct the interest expense on your tax return.
If you have copies of the bills you paid, just go by that total for the payments you thought you were making. The problem is that the original mortgage amortization schedule will be different at this point because you missed some of the originally scheduled payments, so the actual interest expense is higher than anticipated.
If I were you I would contact the mortgage company and ask for a new revised amortization schedule that shows how your payments were applied. Applying your payments to the oldest payments first would be the correct way for the mortgage company to handle it.
However, you should be able to obtain a revised record of how your payments were applied and what your current balance of your outstanding principal is.