Even though there is not a drug that can be used in cats to treat the heart worms, from what I found online, it looks like it can be managed through prednisolone and feline asthma treatments. The goal is to keep the cat healthy enough to fight the worms naturally and out live them. They do have a shorter lifespan in cats than in dogs. I have never had any experience with heart worms, so hopefully there is someone on this list that can offer you more advice.
The feline diabetes is something we can help you with. It is a very manageable disease with a low carb/high protein diet, hometesting and insulin. For the diet, you want to feed either canned or raw food - no dry food. Dry food is high in carbs. There are a couple brands that are lower in carbs than most brands and you can use them while trying to transition your cat to canned food if needed. One we recommend is Evo Kitten food. For canned foods, many of us feed our cats either Fancy Feast or Friskies. The pate flavors are lower in carbs; try to avoid anything with gravy. You also do not need to feed your cat prescription food (even if your vet tries to sell otherwise) and most cats stop eating it anyway.
Hometesting means using a glucose meter to test a drop of your cat's blood before every shot of insulin. You can use either a pet or human glucose meter. Both work well. The advantage to a human meter is they less expensive to buy and use and the strips are available at most pharmacies. Pet meters are expensive to buy and use and the strips are mainly available through your vet's office. You will test a small drop of blood from your cat's ear. Don't worry, it sounds worse than it really is. Most cats are not even bothered by it. We can help you learn to test.
There are several insulins that work well in cats. Lantus and Levemir are human insulins. If your vet prescribes either one of these, ask for a prescription for the pens instead of a vial. With a vial, the insulin will become ineffective long before you will use all of it. The pens are packaged in smaller containers and there are 5 pens in a package. Because they are in smaller containers, you should be able to use almost every drop. One package of pens will give you enough insulin to last about 9 months or more, depending on your dose. Also on the Lantus and Levemir websites, the manufacturers usually have discount coupons for the pens. Prozinc and PZI are pet insulins and you will order these through your vet's office. All four are long acting insulins and cats respond well to them. You want to avoid short acting insulins such as Humulin, Novolin, Vetsulin and Caninsulin. They have steep drops in the blood glucose levels. With any insulin you want to "start low and go slow". This means starting at a low dose such as 1/2 to 1 unit twice a day. If an increase in dose is needed, it is determined by monitoring the current dose for at least one week before determining an increase is needed. If the dose needs to be increased, it is only increase by 1/2 unit. It does not seem like a lot of insulin, but it is amazing how much affect it can have on a cat's glucose levels. Too much insulin can cause hypoglycemia and that could be dangerous.
Start reading the info on this site. Ask any questions you may have. We are here to help you.