Need help with my monthly IRS tax debt payments!?

I have been dealing with the IRS for over 2 months now about my 2009/10 tax debt. They want me to make per month payments of over $400 a month, which I can only afford by cutting out any bill that isn't mortgage, electric, or small living expenses. I understand that I owe, and I am willing to pay, but they are basing these payments soley on the assumption that any extra money that isn't going to a necessity should go to them, for 60 months. As a single parent of two, that isn't acceptable.