S., S., S............reading your story literally brought tears to my eyes, because I have been through this already with one child (step-son 22 now) and am going through it with my now 18 year old step son. I am blessed to have a beautiful, (mostly well behaved) daughter who is also 18. And, I have a son who is 15. So, like you, I am on my second marriage--first one the only thing that was right was I got my two beautiful children!
So, some sugesstions for your problem. This is what we are doing with our 18 year old, who still lives at home and since he had his 18th b-day, he thinks, and has actually said "I'm a man now, I can make my own decisions"......and, my husband and I like you, support him financially, pay for his auto insurance, cell phone, etc.........
With the first step son, it didn't matter how old he got, as long as he lived at home and wanted to reap the benefits of living at home, he had to follow our rules. We are imposing the same rules on our 18 year old, and, trust me, he doesn't like it. So, this is the deal they get with us and it actually worked for us;
No curfew, but constant contact about where you are at, who you are with and what time you will be coming home. The first time they don't come home and don't call--we take the cell phone & the car (since we paid for it and are still paying for insurance on the vehicle). Of course, along with the immediate stop of cash flow. This form of punishment has worked with our older boys. We only had to do this once with our oldest and he never stayed out without calling again. They also know that they are in deep doo-doo if they drink & drive home. So, we have an open door policy--If you have been drinking and want to come home call us (no matter what time) and we will come get you. Yes, they will be verbally reprimanded for drinking, period, but all help stops from us if they even once drink & drive.
So, maybe you need to tell your daughter (as hard as I know it will be for you) that if she really wants to be that independent......give you her keys, her cell phone, and whatever else you may be paying for and tell her to start walking...........I promise you that it won't take her long to realize that as long as she is still at home, she has to follow the rules! Or, in the alternative, get her own apartment, make her own car payment, GET A JOB #1!!!!!!!!!!!! We also emphasized the point that if they don't call, we, as parents, have to stay up all night worrying about them. So, basically, with our teenagers, it boils down to respect, morals and responsibility.
I would also sit down with her and have a discussion about her boyfriend situation and ask her to look into her future and really think about whether or not she wants to live on welfare or be a productive member of society......
Good luck and let me know how you are doing. God Bless you and your family!!! Don't you wish parenting came with a handbook??? :) I sure do!! I have 2 graduating high school next month...........uggghhhhhhh. Oh, and I too, found the perfect man for me the second time around! :)