L.C.
Sounds to me like you are doing exactly the right thing... and that she is pretty much a typical 8 year old.
1.Most children don't blindly follow instuctions. Break down exactly what you want her to do in small steps, and have her repeat it, like you are already doing. Then stay focused on her until she follows through. I can tell one of my kids to go brush their teeth, they say OK, then if I walk away it doesn't get done. I have to stand there and watch them walk to the bathroom so I know it's actually getting done right away before they "forget". I remember going through the same thing with my parents when I was a kid and I can hear my dad's voice in my head, "I know... you forgot." It doesn't make it any easier on me, but it helps me put things in perspective that they are normal!
2.You said she wants to wear what she wants and I wonder why you have a problem with that. My daughter (who is 10) doesn't OWN any clothes that she isn't allowed to wear! If it's an issue of her wearing a skirt that's too short, for example, make it disappear from her closet-- problem solved. When my daughter's shorts get too short (because she has grown), those shorts magically disappear from her drawer and it's time to go shopping for new ones. If it's an issue with school dress code like wanting to wear a shirt with no sleeves, then that is a SCHOOL dress code issue and she is not allowed to break those rules-- it's not YOUR rule, it's the school's rule-- get the teacher involved if necessary.
You didn't say if her mom is still involved in her life or not. To me, that makes a big difference in the role you take. If you are the only (or best) role model in her life, it's more important that you set and keep boundaries than it is to have her "like you". I think a parent is letting a child down if they let them get away with being disrespectful and/or let a child walk all over them, so I think you are absolutely doing the right thing. Keep up the good work, and know you are not alone. As moms, we are ALL going through the same thing, whether our kids are blood related or not