I'm not going to hold up my offer to pay half of a girl's rent
Why are going back on your offer to help pay her rent?
She doesn't even work!
It's not really reasonable for her to expect you to pay her rent, especially if she's not working. But it's also wrong to go back on your word. Maybe you could help her find a job so she can pay her own rent.
I don't feel it is my job to do that, I have better things to do with my time.
I get what you are saying but you should stop making promises you have no intention on keeping. You can get people in some bad situations  if you keep flaking on your obligations.