It's an OLD saying too (Benjamin Franklin) that "Those who understand compound interest earn it, and those who don't, pay it."
If you take a few minutes to set up an automatic account with your bank (a Money market account, for exmple) they will automatically draft your savings/checking for what ever addional amount you bring home on your check. (like $900 in your case) and place it in an interest bearing account. Then at the end of the year, you will have "X" amount in that account. Now, in February you can send MORE money to your mortgage company. Not sure where the "old wise tale" is wrong here?
True, most folks will just spend the extra $900/m, but most would also take the $10-12K and go to Vegas once a year, too. (or big screen TV, Prescision turbo, GN1 heads................) You have the mental discipline to do the opposite, though. GREAT job! But now is the time for you to start using your money for YOU! It's simple, really! Talk to your banker, tell them what you are planning, they can set it up. There are even better places to save your money, too, but I won't get into that right now.