Diabetes isn't great. Never mind having to cut down on the sweets or avoid them entirely; it's a potentially serious condition that requires a significant lifestyle change. The American Association of Diabetes Educators' (AADE) AADE Diabetes Goal Tracker app has been designed with this in mind, and is available to help diabetics make the changes they need to keep themselves healthy.
It's not really a medical app, but rather one that lets users set personal goals that they can share and compare with others. Select a category (Healthy Eating, Taking Medication, Reducing Risks, etc), create a specific goal, set reminders if necessary, then track your progress as you work to make something like cutting back on soda a natural habit.