I'm not going to pay for two months of groceries when I haven't been home for a month and a half.
Is there a reason you refuse to pay?
I don't want to spend that money anymore. I'm tired of it.
That can be frustrating, but you should honor the agreements that you make.
What if I have my own family and want to do something for them?
You should reconsider whatever agreement you have if you need to support your family, but only after you have honored your current one. It's not okay to back out of something you agreed to.