Diabetes screws up EVERYTHING. Not only is feeling bad a real drag, but it DOES affect your "moods". My wife almost divorced me before I was diagnosed.
So YES you will always have to deal with this -- UNLESS he gets his diseases under control.
And here is some added incentive -- Diabetes affects the sexual functions, and men more so then women. He will first be affected by Erectile Dysfunction, probably caused by poor blood circulation caused by diabetes. As the diabetes eats into his nerves, he will lose the ability to ejaculate, and eventually will become completely impotent.
If THAT isn't enough to get him back on the right track, then YOU need to leave him for someone who is more concerned about his ability to love you even though he has a serious disease.
Diabetes canNOT be cured, but it CAN be controlled. Those sexual and mental problems can be lessened, maybe even eliminated if HE works with his doctor to get his disease controlled. Not to mention the threat of heart attack, stroke, high cholesterol, Alzheimers, kidney failure, dialysis, and amputation are reduced or eliminated.
But I can speak from experience -- the love (emotional and physical) of a good woman can be an extremely powerful reason to seek help with this disease. Don't give up on him just yet. He may need some urging, maybe even some threatening, but if YOU give him a reason to change, there is a good chance he will.