Is she in highschool?
If she isnt still in high school, tell her she needs to pay rent or leave. If she is going to pay rent, seh is a tenant, and will obey the rules of the house. If not, then good luck, you love her, and wish her the best, but she has 30 days to find a new place. You will not tolerate her attitude, and she has no choice but to respect you, as it is yoru house and your rules. If life is so horrible, she can leave.
If she is in highschool, then tell her she can start savign and start looknig for a place to live, as upon graduation seh is not to be living in your home any longer. If she wishes to stay, fine, but it is your house adn your rules, and seh will be paying rent.
teh fact is, this girl is an adult now, legally. You do not ahve to deal with the disrespect. Eventually, I am sure she will outgrow this, but for now she is 18 and knows it all. You have to put yourself first. you have given 18 years to this person, and if seh has no respect for that then show ehr to the door.
I know that sounds harsh, but enabeling ehr will not fix the problem. Stand your ground and show her you are serious. After all, you are the mother.