Exactly what Caoimhe said. When you get a horse you know that it is your responsibility to care for it and if you aren’t in the situation to do that then you should not own a horse. I know it sounds harsh but that is what comes with owning a horse. Knowing that your horse is sick and not doing anything about it is neglect. I understand that you want to help, but like you said you are not in the right position for that at the moment. I would try find someone that can take your horse and properly care for it as soon as you can IF they are going to live a good healthy life. Like caoimhe had said about euthanasia is exactly what I agree with. You have to think about your horse and not yourself when making these decisions. This isn’t supposed to be mean, it’s just the harsh reality of it. Your horse needs a vet, you have to find some way to get one.
My reply is going to seem a bit heartless, but if your horse is at a stage where it needs a vet and you are unable to get a vet, it is possibly best to look into euthanasia.
When we get animals, we get them with the understanding that they can't care for themselves. They can't do their own teeth, they can't trim their own nails, and they can't take themselves to the vet when they're ill. Instead, they rely on us to do that. It's our responsibility to keep them comfortable, and it's our responsibility to make the decisions they can't, even the difficult ones. There is no benefit in keeping a horse in suffering, and it impinges on their 5 Freedoms.
Nobody on this forum can help you with this. Complications of diabetes are far beyond the knowledge of your average horse owner, and you'll likely only get dangerous advice. We are not vets. Majority of people aren't even professionally working with animals, and those that are, you have absolutely no way of determining if they are lying or not. The only person who can help your horse right now is a vet.
I agree with Caoimhe. You can search and get a lot of online help but in the end you will most likely need a vet to come out and check her out professionally. I would ask your dad if he could pay for whatever you can’t afford at the moment and pay him back, like a loan. If your horse is sick you really can’t keep putting it off. I understand your situation and it’s a hard thing to get through, but you really do need to do something about it as soon as you can.