Twofish, it's not so much tough love as it is setting boundaries and refusing to play their games. You admit that they manipulate you. Well, you have to refuse to be manipulated. You must be consistent with this. If you back down just once, that gives them the idea that they can make you back down again.
Setting boundaries is healthy for you. It doesn't mean you don't love your daughters; it just means that you refuse to allow them to make you do things you don't feel is in their best interests. Something inside you caused you to make the decisions you did today. Something told you that you didn't want to play this game anymore. That's a good thing!