Okay, I'm approaching thirty and have, over the past few months, been trying to set boundaries between my family and me. My mother assumes that my absence in her life is a result of my not taking care of myself, and she believes that the only way I will take care of myself is if she monitors me at all times.
I am not diabetic, but I can relate to the overbearing mother. When I first got married (at 17), my mom drove me completely crazy. She just couldn't believe I could take care of myself, so she was constantly on me about all sorts of things. Honestly, I ended up letting the relationship drop quite a bit and rarely talked to her for several years (we lived in the same town also, and still do). Eventually she gave up trying to "mother" me and we became closer again (I'm 39 now, have three kids, and am still very happily married to the dude I married at 17).
I know this isn't the same situation, but as you live on your own, you can choose when to pick up the phone or visit her. As for me, I'm thrilled when my Sarah takes responsibility for her diabetes and I consider it my job to guide her to the point where she's 110% independent - as I will not always be around to take care of it for her, and she will eventually grow up and need to take care of herself. If you haven't had any health problems, it sounds like you're doing a great job of managing your health, so I agree with some of the other comments. Let your mom know that she did such a great job training you when you were younger that you're ready to take care of yourself now. And let her know that if/when she brings up your diabetes you will need to politely excuse yourself from the conversation.