Ugh. You need a hug. And some ice cream. :D
FWIW, it's not necessarily a symptom of her being a foster child. My daughter will be 18 at the end of this month. And we're coming down off of two years of sheer hell. No lie. No exaggeration. She was defiant, out of control, sassy, beligerant, dangerous (yes, dangerous), and everything rotten, with the execption of promiscuous and doing drugs. That really wasn't her thing, but like anger and attitude? OMG, please somebody save me!
In the past year or so, we've been working with psychiatrists, counselors, and finally an endocrineologist (I know...what? An endocrineologist???) and things are starting to straighten out. In her case, she had some medical issues that were compounding the "rotten teenager" syndrome. Her whole life, she was a high achieving, kind, courteous, thoughtful person. And then, for about 2 years, hell on wheels. And now, I'd say for a solid 6 months, maybe more, she's kinda grown up. And your foster daughter will, too.
I, too, was trying to be the peace keeper, the forgiver, the "I love you, but not your actions" mom. And she took full advantage, believe me. Someone said your daughter is trying you, to see "Do you love me enough?" and that's totally true. What worked for us is for me to stop giving second, third, fourth chances. One time, she wanted to go to a dance or something after I'd signed up for the School of Rules And Consequences. So, she kept casually mentioning it. And I said "OH, that's great! Just make sure i have proof of your grades before Friday!" and she kept mentioning it, I kept saying "OK, great!" and when Friday rolled around she "forgot" to get the grades. And I basically told her "I'm sorry, I've not been doing a good job enforcing what I've told you I was going to do. That's not fair to you. I said you could go, as long as you had proof of your grades. You didn't do that, so I must follow through on what I said. It's my job as a parent...I would be doing you a disservice if I didn't." In the past, I would have taken her word, only to find out that she was missing 4 assignments in English or something, LOL! Our school has an online site you can check, and most of the teachers use it. So, I offered a solution. Then later, she came down and said "Oh, I forgot my password, and I tried it too many times and it locked me out..." and I *KNOW* she wanted me to feel sympathetic and go "Ohhh, you tried, so sorry...let's get you ready for the dance. NEXT TIME bring the grades home, ok?"
Heh. But I didn't. I just said sorry, and later did some sleuthing on my own. Because parents have their own account to view their students' grades. While not all the grades were on there, two of the 4 classes she was in were, and one she had like a D or something in it. So, I printed it and put it on her desk. The-end.
So, with your foster daughter...what happens if she's 16 and staying at her boyfriend's? Is she considered a runaway? Delinquent? How old is the boyfriend? Personally, I'd call the cops if he was over 18. I'd use the systems in place to put some pressure on her to straighten up. Does she come back to the house when she needs money-food-laundry? There's gotta be a hard talk about you're welcome to live here, but if you're grown enough to go live like an adult (with the boyfriend) then you can support yourself as well. Which to me, means her giving back the cell phone (if she has one), the gas card, the key to your house, etc etc.
In the end, we teach people how to treat us, you know? How do you want her to treat you? And then be smart about it, and find ways to really let her treat you how you want to be treated. Not to say you should manipulate her like "If you don't do X, then I'm gonna do Y" but more like "THis is what I expect."
It's hard. It's an art. And I think everyone goes through this to some degree or another. <3