How do you get over wishing that this person you are left to care for was no longer in the picture?

And I hate even saying such a thing but we have reached this point and cannot get passed it. MIL has always been difficult to deal with but since she has moved it with us it has gotten 100 times worse if that is even possible. She was in an auto accident (and not the first one) which initiated the move in the first place and now since no one will help her to get another car she is down right nasty to everyone. We honestly do not feel that she is safe to drive and want no parts of providing her with another car. We tried speaking to her primary care Dr. about her driving but she sees nothing wrong with her as she puts it "so she is a little reckless". She cannot afford assisted living or to be on her own so that left us since we are her only family but am now thinking this was the biggest mistake we could have ever made. We have teenages still living a home and they too feel like they are being punished. I am not sure what the next step is but I think we need to act now before it spins anymore out of control. Thanks for letting me vent.............