Hi Callum, I have been a type 1 diabetic for 10 years now from 17 to 27 so I went through a very similar experience. It is really annoying to not be able to join in as much as you used to, the biggest changes for me were cutting back on pizza and junk food, and doing the carb counting.
It can feel like you are not the same person, but a person isn't just the sum of what they do, despite what teenagers think. You still like the same things, even if you have to manage your diabetes around them. I have found I can still do everything I enjoy, I still go to parties and I still drink, I just stay off the pizza & soda and don't get so out of it I don't know what is happening.
My diabetic nurse told me when I was diagnosed "You control your diabetes, so it doesn't control you." which at the time I thought was meaningless drivel to make me feel better, but it really is true.
Hope you can keep on top of it all, and keep having fun, otherwise what is the point right?