I would say if she wants to act grown, let her be grown. Don't constantly contact her, text her saying "if youre going out tonight, let me know you get home safe and have a blast!" That's all she needs.
Do NOT leave the bank of mum open for her and fund her lifestyle, she can get a job, if she doesnt already have one, and fund her own life. Don't drop everything for her and make yourself readily available all the time (unless it's an emergency) You're the parent and you're in control.
You're not crazy for feeling the way you do, you're a worried parent and she's your only child, she will always be your baby but you have to let her spread her wings, try new things and just quietly let her know you're always there for her.
Seeing some of the rude replies on here shock me. Youre doing nothing wrong. She's your only baby and you have every right to be concerned, but her behaviour is relatively normal.
For a bit of reassurance, I went to uni, I went out partying and then I dropped out of uni, moved to a different town with a new boyfriend and lied to my parents for months telling them I was going to uni when I wasn't. When my parents found out I dropped out they lost their minds!
But I'm now 27 with a mortgage, a stable job and a stable relationship (Not the one I got in when I moved here). Let the girl live but don't molly coddle her forever, you may be her mother but she has to learn and she will be grateful for you in the long run.