How to deal a person with diabetes?

My partner is suffering from diabetes and he's taking insulin shot twice a day and pupping several pills everyday. Does his being in this kind of health problem affect his mood? Everyday, I feel so stressful anticipating on what mood he gonna be in. Sometimes, he's jolly and happy but often he's very quite and just like out from the universe. It is really taking a lot of my energy trying to understand his sudden mood swings that I am almost giving up. And its now over a year that things has been like this. I love him and I swear to God that I want to serve and take care of him even for a lifetime but I dont know how long I can tolerate his erratic behaviour. I need someone who can enlighten my mind regarding this problem. thank you so much for any advice i may hear from somebody.