Have you tried sitting her down and just asking her to talk to you about how she feels? Maybe the fact that you and her diabetes team are having conversations about her "behind her back" might be making her feel a bit rebellious? I know it's so hard, but could you try to put less pressure on her to do the tests all the time and have the perfect results? Maybe tell her you have no idea how hard it is for her to deal with this on her own, but that you are there for her and are trying to help as best you can.
I never really wanted or volunteered to speak to anyone about how I felt (which was bad!) but I think if someone (not neccesarily my mum or dad) had actually asked me "How do you feel about all of this?" instead of telling me "you must do this amount of tests and at these times" and then telling me off and treating me like a naughty child if I missed one or didn't get a good test result, I would probably have burst into tears, had a good cry, and then been quite honest and said how I felt like my body was betraying me, like I wasn't in control of anything, how much I just wanted to be a normal teenage girl, how unfair it all was!
I don't know what your relationship is like with your daughter, but I never felt that close to my mum and like I could just talk to her, which is why I suggest you asking her!
(I am 26 now and still feel like doing the above sometimes!)
My doctor once asked me to up my number of tests from 4 to 8. When I got a bit upset and said I didn't like doing them, and didn't know if I would be able to do that many, he patted me on the knee and said "Well, it's not that difficult, your nurse team and I all pretended to be diabetic for a week and did the blood tests. Obviously not the injections though, hahaha." And all I wanted to do was punch him in his stupid face! How dare he compare a weeks worth of tests to living with this for a lifetime??
As a teenager, everyone is a bit emotionally/hormonally turbulent, the diabetes adds to this so much more, I was a terrible diabetic at that age and I am still struggling for good control, but I only really took charge of it myself when I realised that no one else was going to suffer the consequences of not monitoring my blood sugars and taking my insulin correctly - and as pneu said, when my doctor started talking to me about babies and pregnancy and how good your control needs to be. That for me was the real kickstarter for sure.
Sorry if any of this sounds patronising or inappropriate - but it's what I wish someone had told my mum when I was younger! My mum was a nurse, so every clinic I felt like she and the nurses were talking about me behind my back, and the doctor would explain everything to her instead of me because she understood it, and sometimes he'd even say, "oh well your mum can explain it you later" if I said I didnt understand something or needed help with it. It made me feel so furious, and the only way I could get "control" over my body again would be by being naughty and not testing. Crazy teenage girl logic at work there!
Good luck Jackgard - and again, echoing Snodger, I'm sure we'd all be willing to help by speaking to your daughter if you'd like us to.