Here are three possible solutions.
The financial institution that services your mortgage is required by law to supply this information. If you haven't got it, call them.
If you have your receipts from the year, the interest is provided there, and you can add them all up. If you have the last one you can back track by looking at what you paid in principal, interest and full payment. It take a lot of math, but it can be done. If you have last January's bill, you can do the same and work forward.
If you have you mortgage information from the beginning including Mortgage amount, rate, number of payments, and know how long you have had your mortgage, you can use a free mortgage payment calculator on the internet and you can plug in the numbers and you will get all the payments amortized out and you can select the twelve payments that fit within the appropriate year. The principal will be displayed for each payment.
I hope this helps.