I am a more recent parent, however, I am (by a long measure) the last of my friends to go down this path and I was a T1-teen, so, on the chance that it may be useful, I am going to toss in my cent and a half. J
First, after a great deal of observation I am almost certain that “nag” is teen-speak for “caring adult.”
Secondly, the teen years are about discovering and managing independence while pushing away from home and identifying and managing limits. I am sure some manage this elegantly, but managing all that goes with being a teen while also juggling a demanding chronic illness can really be an awkward and frustrating affair. Worse, it is unfair and there is generally no nearby easily identifiable role model handy by whom one can even begin to gauge their progress.
My advice is patient, persistence, unrelenting caring and guidance. I say this knowing that it is very likely, very much like what you are doing. Be willing to work with your daughter if she can think of other things and ways that you can work together on managing things as she takes over more of the responsibility for her management.
I suspect your concern is as normal as a teenager’s unyielding belief that they will be fine because they said so… J Doesn’t being normal feel great?!?!?!? <EG>