If you’re committed to paying off your mortgage in 15 years, use a mortgage calculator and enter the details for your loan.
You can use this to determine what your additional monthly payment needs to be in order to pay off your loan in 15 years.
Once you have this amount, send anything above your required monthly payment as a separate check.
Send that check with the instructions to apply it to principle and you’ll be on track to pay off in half the time!