Hey Alex,
I've been dealing with depression and anxiety for over a decade and a half, and as of a few years ago diabetes as well. It's tough, I get it, I understand the need to give up because it feels useless because it's a lot to figure out, people nag at you and don't listen.
Thing is, depression is a big fat liar. It makes us think things that aren't true just to get its way, just to make us feel awful. Thinking diabetes is our fault is another lie. Thinking we can't do anything right is another lie. It feels like the truth but it really isn't, and it's very important to acknowledge this.
Diabetes management is tough, but not impossible. You just have to set some time aside every day to figure it out. Diabetes is part of our lives, and we have to give it its own space and not ignore it. Once it has its own space it can settle in and you can team up to get everything sorted. Think of it like a roommate for life - if you just sit and sulk and don't listen so you know what chores need to get done, you're going to live in a messy house (your body). But if you listen and do the chores that your roommate tells you to do, you can have a clean house! It won't always be clean, there's always going to be some dishes left out, there's always laundry to do, but it'll be managed and livable. As with any new housemate it takes some time to figure out schedules, adjust to quirks, keep in mind how your roommate's doing when you're both sick or dealing with heat, you'll have some arguments and so forth. It won't ever be perfect, but it can be comfortable and managed well so you can coexist peacefully.
Depression is also a serious condition - it's as valid and real as a broken leg, and you deserve proper treatment. Please see your GP and get a referral to a psychologist. Find a therapist whom you click with and then get to work on getting better with your mental well being. It will have an automatic positive effect on your diabetes management too! Cognitive therapy might benefit you because it helps you reframe the situations that drain you and trigger symptoms, and effectively retrain your brain. Try to get on a good sleep and eat schedule (I know, it's awfully hard, but I believe you can do it!) because it really serves a solid function in being mentally healthy.
Remember that you didn't do anything wrong. Sometimes **** just happens. We don't control that, but we DO control how we deal with it, and the very fact you reached out tells me you're a smart person who wants to learn how to deal with things the best way possible.