She needs to learn to be an autonomous adult
Sad Mom - She's older than 18 and doesn't live under your roof? She's not your responsibility anymore. Let me ask you this, if you broke someone's windshield, are your mommy and daddy going to pay for it? Of course not, that's your problem. At this very moment, you need to admit to yourself, "okay, I messed up buying my adult daughter this car and not teaching her the values of hard work and sacrifice but from this moment on, I am not going to continue to spoil her this way." Admit that part of the reason you did and do this for her is to make yourself feel better. Stop buying her all this stuff, you can't buy her love or her gratitude. She needs to change her attitude towards life, not just you. If she's living with this loser, you have to respect that it is her choice and she will just have to learn the hard way, hopefully, she won't get pregnant and put a baby in the middle of all this stuff. She needs to grow up now! Take the car back from her today, now, at this very moment! Not out of spite, out of your desire for her to become a real adult. If you ask her for the car, she will yell and scream and tell you what a terrible parent you are and tell you again that she hates you... yada, yada, yada. Since you don't know where she lives, you will have to use the same manipulation she used on you, tell her you have something for her, a surprise you can't tell her about but one that she will really love and appreciate (its not like you're lying, someday when she does grow up, she will love you for what you did!) Have a tow service ready to pick up the car (let them know you don't have the keys and you will have to show them you are the financially responsible party for that car) and have it towed somewhere until you can get the keys (if push comes to shove, have a locksmith make you a set) and park it in your driveway. If you have your own car paid off, sell it and use the BMW. If not, let your daughter know that the free ride is over. Tell her that you respect that she has moved out, made her own decisions and decided to become an adult on her own. Tell her that by becoming an adult (a teacher no less?? ) she will have to be self-sufficient and responsible. She can take the bus or carpool or do whatever it takes until she can purchase a car of her own. This is the only way she will truly learn to be a TRUE ADULT, age has nothing to do with it. Oh and by the way, your kids should NEVER be your best friends, otherwise, it gives them the idea that you are equals, you certainly are not. You can be very close, be there for them, share lots of good times, but always remember they are your children, not your best friends, keep the parent-child respect intact.
By taking these actions, you have to be willing to let her hate you for all the garbage she's going to go through, but there is a light at the end of the tunnel, and it will only turn on if she is the one who wades through all the garbage and turns it on. Maybe you can make a deal with her, she can buy the car from you (and no, don't lend her the money to do that, let her figure out how she's going to do it), she needs to give you the same down payment they would expect from her at a car dealership (so keep her credit history in mind) and monthly payments should be expected from her on an exact due date. Remember, if you are flexible with her, she won't learn anything. The real world is not flexible either and she needs to learn this today! You feel bad because you have a conscience and there is nothing wrong with that, there is something wrong with having inappropriate guilt though. Having a back bone is not wrong or mean. You will see, things will change for the better, but stick it out, don't give in. This is tough love, I know it works, it did for me, I was that irresponsible, selfish, self-centered little brat until well past my 20's. My parents stopped rescuing me and I had to sink or swim. It was hard work, lots and lots of hard work, but well worth it cause I'm doing the backstroke now. Happily married with three kids and my 15 year old daughter is learning what I didn't learn. She knows that at age 18, if she did well in high school and planned on going to college she is welcomed in our home, as a guest. We will help her out somewhat financially and she may stay as long as she keeps her GPA up and follows our rules. Otherwise, I guess she'll have to learn the phrase, "would you like fries with that...?" Be a parent :)