I can help you understand. First, I have to tell you that it's way more complicated than the explanation I am going to give you. and if you still feel like you want to know more than you should check out a book. You might want to read a book (or an article) on diabetes. Just ask your friendly neighborhood librarian. You might also want to check out some of my sources.
When the general population talks about sugar they are talking about more than one thing. Glucose, fructose, sucrose, are all different sugars. but they all are very similar and a chemist or a biologist would call them all sugar too. The simplest sugars (and there are only a few including glucose and fructose) are just one molecule. All the other sugars are two or more molecules. And as a group, these are also called simple carbohydrates. Complex carbohydrates are compounds made from a combination of at least three or more molecules of sugar.
The simplest carbs, sugar, start being digested in the mouth. The complex carbs need to pass thru the stomach and into the intestine to be digested. The simple sugars can pass thru the lining of the mouth, stomach and small intestines and enter the bloodstream. Complex carbs need to be broken down in the intestines into simple sugars. Then they pass into the bloodstream.
I don't know the minutes it takes but let's just say that simple sugars go into your bloodstream VERY rapidly and complex carbs enter the bloodstream MUCH slower.
To further complicate things you must also consider the role of fiber. Fruits have a lot of sugar but they also contain fiber. Fiber slows down the absorption of sugar in the intestines. Some types of fiber also help recuce your blood sugar levels. and some fiber slows down the secretion of glucose by your liver.
OK, now that I brought up blood sugar levels, its time to explain that. So when sugar enters the bloodstream it increases your blood sugar levels. When blood sugar level rises your pancreases releases insulin which enables your cells to absorb sugar. Also if you have too much sugar in your bloodstream, insulin allows the liver to absorb and store it. The liver will release it if you need sugar like between meals and when you are sleeping. and the liver will also convert unneeded sugar into fat.
So what happens when you eat too much sugar is that it gets quickly absorbed and causes a "spike" in your blood sugar levels. the pancreas then releases too much insulin. your cells and liver absorb all that sugar and then your blood sugar level is too low. When your blood sugar level is too low you might feel light-headed, or weak. might also feel hungry and thirsty. Children because of their low body volume are more susceptible to this kind of stuff. We call them sugar rushes. Give a kid a 24 oz soft drink and he will be energized for like 15 minutes and then he will run out of energy and fall asleep. And yes some people do get headaches. I think the brain gets very upset with us when our sugar levels fall to a low level. I know I get a headache if I to too long between meals.
Most of us who eat healthy usually don't have to worry about this kind of stuff. but it sounds like you are more suceptible than average. So you might want to learn about the guidelines diabetics follow and keep them in mind.
Eat plenty of fiber. Most Americans have diets deficent in fiber. The RDA is 25 grams. the Average american gets 15. And 25 is the recommended minimum. thirty to forty grams of fiber would be a good goal.
Although fruit has a lot of sugar - it also has a lot of fiber and the net result is totally positive. Eat two or more pieces of fruit a day. Almost all your fiber needs to come from vegetables. Yes, they are mostly carbs. but they have very little sugar and lots and lots of fiber. Besides carbohydrates is the fuel that makes our bodies go. No one can gain weight by eating vegetables. Grains are also a good source of carbs and fiber - but it has to be whole grains. Three servings of grains a day is recommended. but many Americans eat six or more. So yes, you have to careful you don't eat too many.