Not a parent, but I'm guessing I was more recently 18 and a college kid than many of the commentators here. So I'm going to try to give you some advice based on her perspective.
- Freshman year, I declared a math major. I was going to be a mathematician or an actuary. I had it all planned out. My dad, who wanted to be a scientist/mathematician but for a ridiculous amount of family reasons was 'forced' not to, was thrilled. You know what I really sucked at? Theoretical math. Listen, I kicked ass in high school calculus and aced my first couple math classes my first semester. I was used to seeing straight As (with maybe the occasional B+) in high school, so it felt like a punch to the stomach when I got my first C. Worse than that, even. And, when you've spent so much time thinking "I want to be X when I grow up," then find out that you're just not that good at it, regardless on how much you studied, it throws you through a complete loop. Well shit, now what was I going to be? It is absolutely ridiculous to expect an 18 year old to know exactly what she wants to do with her life. Sure, there are some kids who have known what they've wanted to be when they grow up since they were little and achieve that goal. The rest of us? Not so much. I'm now in a field that I hadn't even heard of in college.
- College kids are in a really weird place... the world expects them to be adults, but are they? Listen, from 18-22, I thought I was an adult, I was independent, I was on my own. Sure, you have the freedom to stay out until 3am if that's what you want to do, but with parents or loans paying tuition, room and board, dining hall, etc., college kids don't have the responsibility to "pay rent" or "go grocery shopping" (for more than snacks or treats). She thinks she's an adult, you know better, and there's conflict. It happens. Again, not a parent, but I have to imagine nearly all parents have pulled their hair out because their "kid" is acting like an adult, but isn't fully an adult yet. It's normal. She's just as frustrated too.
- Don't compare her to your cousins, or other college kids, at least in front of her. She's trying to figure out who she is now, and you saying or implying "You should be more like Joe," or "Why can't you act more like Jane" is going to cause resentment, and possibly for her to act out. I was one of the 'good' kids, and yet I still wanted to act out against how my parents thought I was. It's all an 18 year old trying to figure out her identity. It's normal.
- She's only been exposed to the mustachian lifestyle - so how does she know the repercussions of spending all of her money? Does she know the excitement of buying something new fades? Has she ever been in a position where she spent her money on something needlessly then couldn't afford something important (or for that matter, couldn't afford something in all, regardless of importance?). My biggest fear, when graduating college, is that I had never truly 'fucked up,' so did that mean I could never fuck up in my adult life? And if I did, how would I know how I could respond? Let her make mistakes. It'll kill you to watch, but she's gotta do it. Let me ask you this - have you never made a mistake? I know, it's different because she's your daughter, and you want what's best for her, and you think you know what's best. But the truth is, letting her make the mistakes and growing from them is what's best for her. And when she makes a mistake, support her the best you can. It doesn't necessarily mean financially, but listen, offer advice if she asks, guide her gently... for the love of god please do not say "told you so," because she'll never tell you anything again. (By the way, when life threw me a huge "fuck you" a few months after college, my parents were my rock, and I will be forever grateful for their love and support during that time.)
- The relationship stuff - you officially get no say. Listen, stick yourself in denial if that's what you want or need, but by attempting to control the way she uses her heart and her body, you're telling her it's acceptable for someone else, especially another man, to dictate that to her too. "But I'm her father, it's different!" Nope, no it's not. And yes, this is from experience.
- Two financial bits here - are you 100% open with how much college costs, and does she fully understand that? The first half you have control over, but (see above for making mistakes) the second half you don't. I had *no* idea how much my expensive 4 year private college cost. My parents, my dad especially, wanted me to go to the best school possible, with the best education and best experience. I turned down a cheaper school that was offering money in favor of my fancy college that offered me none. Now listen, I had a great experience there, loved it, would recommend, but it left both my parents and me loans. I had no idea how much my parents had saved before college, I had no idea how many loans they took out (still don't, really). My dad's goal was to put me through my four years with me being debt free, and he was heartbroken that I ended up having to take out a loan to finish my last year (I was in college during the recession, and he had lost his job for a while). I didn't understand any of this then. It wasn't until I started paying off my loans that I realized how much they sucked. So if your daughter is gung-ho about taking out loans, explain to her what that means, how much she'll be liable for and when. She still might not get it. But that's part of growing up.
Tell her you love her. Tell her you'll support her with your love no matter what. Tell her if she wants to switch from nursing to something else, that's okay, and you can help her figure it out if that's what she wants. (And I do mean tell her all this, using words. I *knew* my parents were supportive of my switching majors/figuring out what I wanted to do, but when my dad actually said it, it was the biggest relief and made me feel so much more comfortable talking to him about it.) Figure out financially what you will be willing to do, and tell her the parameters. Tell her you'll help her figure out how to get a loan and help her apply if that's what she really wants, but you won't cosign (if that's what you want).