Be supportive, but don't be overbearing
Chronic illness is stressful in a relationship. Diabetes is no different than any other chronic disease. Be supportive without making them feel like you are always nagging. Accept that there are things you can do, and also that there are things you cannot change. Support your partner who has diabetes but don’t make them do anything they don’t want to do.
You know that losing weight will help your diabetic partner cut down on insulin injections or make them feel better, but continually harping about them going to the gym or just taking a walk can serve no purpose. Let them think that exercising and walking is their idea, then be happy when they decide to change their lifestyle.