Hi Sadie, I'm 27, so have also left the teenager camp, but I still remember it very well, and believe my situation then had a number of parallels to yours. Therefore, I will share my experiences so you can see there is light at the end of the tunnel. I have managed to take control, and you can too.
I was also diagnosed at the age of 3. Until I went to secondary school, my parents were fully in charge of my diabetes, and my control was exemplary. I will always remember my first day of year 7. One of my new friends offered me a skittle at lunch time. I cautiously accepted, and was delighted I felt no ill side effects. I thought 'great, I can do what I want!' Over the next couple of years my control began to slip. My HBA1C steadily rose, peaking at 10.8. As my control deteriorated, so did everything else. I couldn't concentrate on anything, and my school work suffered. At primary school I was the top of the class. By the end of year 7 I was coming bottom in most subjects. I also refused to admit there was a problem. I became moody and shut off, particularly to my diabetes team. If homework was something to do at the last minute on the bus, noting down blood sugar results was never going to happen. When I was taken to diabetic clinic, I would 'nip to the loo' and make up 3 months of blood test results so I would not get in trouble. My DSN knew I was not bothering. She tried to encourage me by telling me the benefits of good control. When that didn't work, she tried to scare me with the stories of diabetics who got complications. I never cared in front of her. But it was a different story at home. I was terrified, and did not believe I would survive into my twenties or thirties. Every night I would tell myself tomorrow would be a day of good control. Every day I would slip at lunch time, treating myself to sweets because I was unhappy. It was not until I had done my GCSEs that I really started making any effort. My diabetes had become a battle of wills with my DSN. I felt I knew best and she was just getting at me (this was not the case, she tried her **** hardest with me).
When I reached 16 I was transferred to a young adults diabetic clinic at a different hospital. They ran social events for diabetics my age, and I got to meet others in my situation. Everyone else seemed to have better control than me, and I did not like once again feeling like I was bottom of the class. So I started putting in more effort, and over time my HBA1C dropped again.
I think we all need inspiration or an incentive to stay on the straight and narrow with diabetes. You have taken an important step in asking for help. The very best thing you can do is to talk to those around you, and share everything with them. I was terrified of talking things through, and to this day I don't know why. Those close to you will be able to help, and the more you explain, the better they can understand what you are going through. We have not been set an easy task, but you can come through it, and be stronger for it. Best of luck, and I hope my story has given you hope things can change for the better.