welcome 🙂
many people here suffer or have suffered with depression, it is very common for poeple with diabetes to suffer form this, and there are various threads if you have a look around.
Mood swings tend to be linked with swinging blood sugars. I know for me when my blood sugars are high then I am more irritable and have less patience. I try not to be too moody with my partner but sometimes it can't be helped and I am lucky she is understanding.
I wonder if you partner is having problems with their control at the moment? I would suggest talking to them when they are having a good day, with good blood sugars.