Your post broke my heart. Your cat is very lucky to have you.
One month is a very short period of time and with a sick child in the mix! You are doing fine. Please don't be so hard on yourself, but, yes, you will have to make the commitment to act as if your cat is child, and treat him for his diabetes as if he is a child who can't tell you what is going on. Remember, even though he may not understand this, just like a child, a pin prick is a far less damaging and painful thing than the ravages of uncontrolled diabetes, in the long run.
To make you feel better, cats are said to be very resilient when it comes to diabetes. It may take you quite a while to get it under control or it could be very quick, but cats in the wild are from the desert and sometimes don't eat for days and then gorge. Their bodies can take a lot of abuse, and I'm sure your cat will be just fine given your love for him.
Everyone has given you good advice, but I would like to add this: hydration is very important for a diabetic cat. They tend to get slightly dehydrated and over time this causes other medical problems. There are many tricks to getting a cat to get enough water.
1. Getting the cat off dry food - cats are not designed to eat dry food. They don't usually drink much water in the wild -- they get hydration from their food. Switching to dry food and starting insulin at the same time can be dangerous. One tactic is to do one or the other, start testing, and then start the other change (insulin or wet food) and watch carefully for how the insulin affects your cat. Given that you have started insulin, you may want to get some data on your spreadsheet for a week or so, and then switch the food. Other people here may have other suggestions for this issue.
2. Mixing water into the dry food. As much as the cat will allow you to mix it without refusing the food. Wet food isn't quite as wet as it should be when the cat eats it.
3. Getting a cat fountain. I have found that the cheaper ones make a horrible sound, and you want the fountain where you will notice when your cat drinks. So, getting a good one is important. Mine is a Petmate, and it's several years old. You can wash it in the dishwasher, but if it gets slimy you will need a baby bottle brush and a toothbrush to clean it. I put a tiny drop or two of bleach in the water to keep the slime content down.
4. Checking the poo. Fun! Fun! I check my cat's poo to see if it is too dry, but my cat has Chronic Renal Failure all ready from being dehydrated too long. You may not have to do this very long if you see no issues, but if you do notice it is dry, and you can't get the problem fixed, I would recommend a trip to vet for some tests. Your cat is rather young, so this is less likely to be an issue for you.
This is very, very important: Get your hypo kit ready:
http://www.felinediabetes.com/FDMB/viewtopic.php?f=14&t=1122 and
http://www.felinediabetes.com/FDMB/viewtopic.php?f=14&t=2354
Choose a good wet food:
http://binkyspage.tripod.com/CanFoodNew.html By the way, this table is in the process of revision due to the fact that many pet food manufacturers have changed their formulas. Pumbaa's posts about the new food charts.
Post here what type of insulin you are using, what your cat's BG was a diagnosis and what is has been recently, and how many units you are giving and how often. You mentioned 2.5 to 3 units. Is that twice a day (bid)?
Finally, get a spreadsheet set up. Can someone help her find the link for this? Thanks you! I can't find it at the moment.
Good luck.