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Hi and welcome to Just Answer! I'm happy to help with your tax questions. Feel free to interject at any time if you need clarification.
It won't be a problem for you to deduct the mortgage interest you pay to your mom, but please be advised that for you to be able to deduct the interest you pay to your mom, she will need to report it as income on her tax return. If she reports it, you will be able to get a deduction for the full amount of the interest you pay in any given year, including the back interest.
The fact that you did not have a formal document drawn up from the beginning will not affect the tax deductibility of your interest going forward. Draw up a document now, specifying the payment schedule, interest rate, etc. Specify in the document the original date of the loan and that you are paying the back interest in this year.
One caveat--please make sure that the interest rate you are ruling corresponds with the long-term applicable federal rates (AFR) in existence at the time the loan was made. A list of AFRs by month can be found here.