This is a common issue. I have tutored many students for SAT prep who dropped off of their IEPs in high school against the wishes/advice of their parents, teachers and guidance counselors because the resource room is called "the retard room" by other kids (I hate writing that word but that's what the kids say).
If at all possible, I would have the IEP amended to include any important accommodations that really count such as extra time or a separate setting for standardized testing and would acknowledge that she's trying to manage without the other supports and those can be revisited as needed.
Then let her know that you are going to hold her to the same standards for conduct and academic performance that you would if she was receiving her services, whatever that would mean for her. For example, perhaps it's that she gets an 80 or better on all tests and quizzes, turns in 80% of her homework on-time and earns a B- or better in all classes. If she can't maintain those goals, the enforce agreed-upon consequences (no socializing for the weekend, a limit to extra-curriculars, etc.). If she can meet those goals without the supports, then great! If she really can't, then perhaps she'll reconsider availing herself of support outside of the school setting. There is a stigma and shame to SPED services for some kids, while things like "tutoring" and "therapy" are seen as OK.
I'm surprised, honestly, that it took you almost 2 years to get her to a counselor outside of school to figure out the problem, but it is what it is. I'm sure that in hindsight, you would have gone that route sooner.
You need to step back and step out of this and stop making it a power struggle. Sure it's obvious to us that these services are valuable and to her benefit but if she won't avail herself of the help, she'll have to learn the hard way. I posted a while back about frustrations I was having with my 16-year-old son, who is on an IEP (he actually doesn't mind going to the resource room) and has ADHD and basically refuses to participate in the self-care needed to manage his ADHD, which would help with his learning disabilities. Won't take medication, won't eat breakfast or a healthy lunch, is spotty with taking supplements, doesn't maintain an agenda or assignment notebook, etc. I said in the post that "failure wasn't an option" and was taken aback that several people responded that "sure it is" and that if his poor grades mean going to summer school or repeating a grade, then so be it. That was a shocking attitude to me but it was a helpful one too. They're still kids, and they're going to make poor choices, and they're going to make things harder for themselves than they need to. They might not graduate on-time, or might need a few years after high school to figure out their job, vocation or college path and really, that's not the end of the world.
If I were you, I would try for clean start with your daughter. Put the choice in her hands. Let her know that while you see the benefit of her services and hope she avails herself of them, that you obviously can't make her choose them and trust that she is capable of having a voice in this decision. Work *together* on her goals and get specific on what she needs to do to get there. Does she want to go to college? What kind of school? The college board website or any school's website will have an admissions profile. If she has an idea of some schools she'd be interested in, visit the admissions page together and look at the academic profile they're looking for so she knows realistically what it will take to get there. If college isn't realistic, is there a vocation she wants to pursue after high school? What kind of skills and training are required? How do those mesh with her interests and abilities?
With kids this age, it can help them to have a concrete picture of where they want to be after high school, what it takes to get there, and whether or not they're on track. If there is a gap between the track she's on the the one she needs to be on, she can be a part of the conversation about how she's going to close the gap.
Good luck as you deal with this - I know it's hard but at the end of the day, you can't make her want to receive the help that is being offered.