I am a recently diagnosed type 1 diabetic, hospitalised on my 29th birthday earlier this year. Not having received it as a child it is difficult for me to add to the post, but I do think that I can add some value by elaborating on exactly how tricky glucose monitoring and insulin dosages are.
Type 1 diabetes is uncommon in comparison to type 2 diabetes, if I remember the numbers correctly, it is about 10% of diabetics that have type 1. It is an autoimmune disease, meaning that your immune system, for some as-of-yet unknown reason, attacks your pancreas and destroys your ability to produce insulin. There is no medication or treatment other than injections of insulin into your bloodstream for the rest of your life. Personally, I had deteriorating eye sight over the course of several months and finally a very sudden urge to drink large amounts of sweet drinks and water.
As a type 1 diabetic you need to monitor your glucose level several times every day. The most common way to do so is to prick your finger and put a drop of blood on a one-use testing strip that goes with a digital monitor [1]. This is remarkably easy and you get instant results. It is however a relatively new invention and prior to this you had methods like urine test strips that gave far less immediate and accurate results. The most modern monitoring available would be continuous glucose monitors. They are essentially a needle with a sensor attached to a patch that you attach to your skin. This sensor then sends a signal to a device that you carry with you and that can warn you if your values are too high or too low. It is however not widely available in all countries, partially due to the cost.
[1]: https://en.wikipedia.org/wiki/Blood_glucose_monitoring
So, about dosages, how do you do it? Well, every person is slightly different, but there are tricks. You can use the same portion size of carbohydrates every day and keep your dosages fixed, or you can attempt to guess the amount of carbohydrates and adjust the dosages accordingly. What happens if you overdose? Nothing immediate, but over the next few hours or so you will start to feel lethargic, sweating, act "drunk", and in the end loose conciousness unless you compensate by taking in additional carbohydrates. As you grow older you may loose these signals and falling too low becomes increasingly risky. What happens if you underdose? Nothing immediate, but much much less immediate than with an overdose. You will have high levels of glucose in your system which gradually will wreak havoc on your eyes, feet, kidneys, etc., there will be consequences further down the line. This is one reason why getting insurance coverage as a type 1 diabetic is almost futile as pretty much anything could arguably be caused by your diabetes.
To add to all of this your dosages will vary due to what kind of food you are eating, carbohydrates from chocolate acts slower than pure sugar due to it being coated in fat that slows down the digestion. If you are sick your body is likely to be much more difficult to read and your dosages may change radically. Add to this that your immune system is significantly weaker as a type 1 diabetic. Dosages also vary depending on whether or not you do physical exercise, your stress levels, and more.
I really want a book describing all of this to me from the bottom up, molecular chemistry and all, so that I can better understand my own disease. For now I am gradually building a mental model from experience and picking up bits of pieces by reading.
There is hope in stem cell research and computerised systems that automatically read your glucose levels and inject an appropriate amount of insulin. But we are not there yet.