Hi,
@Dude26 and welcome to the forum.
Diabetes can lead to a few lacking minerals and vitamins, which can - in turn - lead to tiredness, grumpiness and whatnot. But, from the little you've said, I suspect that your diabetes
is the cause of your change in mood in that, maybe, you're still quite angry about the diagnosis, the complications, drama, discipline, self-treatment and limitations of it.
As with any life-changing diagnosis, you need to come to terms with it - a process much like grieving - and that can be really hard to do alone. The first thing I'd do is talk to your partner about it - how you feel about your diabetes, as it seems that's something you're avoiding doing at the moment. Let her in on what's happening to you and how you feel - it's incredibly difficult for her, too, especially if you're not telling her what's going on.
And then I'd look for a bit of help and mental health support. There are services that will help you process and come to accept your illness so that you can live without this ball of anger which is big enough to affect your daily life and relationships now.
Of course, I'm just some words on the internet and I don't know you at all. But from what little you've said, that's where I'd put my money.
Good luck and let us know how it goes, eh?
Sock x