Anna, a month from now, you'll have had diabetes for ten years. That's an incredible accomplishment, and one that you should be extremely proud of. It has made you stronger (remember, it changes us but in a good way). The girl who's turned against you isn't strong at all, that's why she needs so win many other people on her side. It takes ten of them to be as strong as one of you. Keep your head up, and don't let them see that it bothers you.
In reality, it does bother you, and that's OK. Diabetes is tough, and being a teen is tough. Doing both is damn near impossible. If you need to take time and let your emotions out, go right ahead -- we all do that every once in awhile. Crying relieves stress and makes us feel better. If others don't get it, and you need time alone, take it (that doesn't mean to stay home from school, though!). Try to stay strong in front of the bullies though; if they can't get a response out of you, they'll probably give up and move on to someone else.