@Ali Akin - with the proviso that almost everything going on with the body of a 50 odd year old is totally different to someone approaching 20, my starting point was definitely about trying to understand the boundary between fatty liver and insulin resistance and diabetes (clearly T2DM, but I wasn't so clear about even that at the time, I was totally clueless) - but - for what it's worth, let me lay out a year of learning in a paragraph or two - I'm always trying to be conscious that I could be wrong about things, but there is quite a lot that seems to me to be uncontroversial.
The first thing to consider is that non-alcoholic fatty liver is relatively new - up till about a generation ago - all fatty liver was considered to be a consequence of alcohol, and one step towards cirrhosis. Given that the liver uniquely has to metabolise both alcohol and fructose, that's a reasonably big clue right there.
I previously laid out all the ways you need to consider the ways that insulin affects the body - but it's just as useful to totally over-simplify and think of it as being the master arbiter of energy management, and that it's triggered in response to foods that easily break down into glucose - so one of the big ways it manages energy is by clearing out glucose into wherever will have it by any means necessary throughout the body - but since there is a kind of cascade of possibilities, that first means around the liver itself, in large muscles, and fat storage cells (first turning it into thing that can be stored in these places).
Sugars and Starch - too much blood glucose - insulin response - less blood glucose - more glucose around the body, starting around the liver.
As we use energy - glucose being consumed in the cells (not exclusively - the heart runs mainly on fat whatever else is going on, for example, but it's always a special case) it gets replenished, starting from the most easily converted storage, the glycogen in the liver (think animal starch) - of which we typically have about a 24 hour capacity, in the liver and muscles.
So - you end up with a kind of "big bucket - little bucket" concept of the body - we fill up the little bucket fairly quickly if we eat sugars and starches, (the liver, short term storage) and then we have to fill up the big bucket - the rest of the body. When we use energy, we use it from the little bucket.
Almost everything then can be thought of as the answer to this question - what happens when we eat another meal before the little bucket is empty?
Your genetics clearly play a part in the answer, because the answer depends on what the food is, and what your body does with it, but for most of us, the answer is that the little bucket is already full, and we keep topping up the big bucket - and then the big bucket starts to over-fill. For some that means putting on weight, for others T2DM, for others all sorts of inflammatory conditions and brain disorders .. but to explain any of that you have to get much more complicated than "big bucket - little bucket".
However - the body can handle this for a long time, you could be talking about 20 years before you even notice an increase in blood glucose, which is where fructose plays a special part.
In what I understand a flexible metabolism to be, your digestive system is in fairly complex coordination with your fat storage and brain, and you absorb nutrients (mainly from protein and fat together) and store them, and use them, and the liver does the fine tuning, also having a direct line to the brain. In diets dominated by surgars and starches, everything (more or less) goes through the liver and it does all the heavy lifting (as described) - but when you have fructose - that; only the liver can deal with, and it can only turn it into fat and store it locally.
That - cramming of fat into storage around the liver - causes the cells to swell up like balloons (fat storage cells, they are meant to do this and can become really huge, comparatively). This is what visceral fat is, and why apparently thin people can have fatty livers. Although fat cells can cope, the danger comes from their coping mechanisms to this swelling. Firstly, they literally push back - like a balloon. How this happens is quite complicated, but the analogy works - this is direct resistance to insulin, like a balloon gets harder to blow as it gets bigger. Secondly - as it gets bigger, it gets further from blood supply, and starts shouting for more blood vessels closer to it - this is the cytokine response, and is literally chronic inflammation (again, directly - nothing else but that fructose). The third thing is also more complicated, but essentially you have more fuel in the cell than the "fire" needs, so the way that energy is produced is altered - if you think of putting too much wood on the fire, that's close enough, and you get too much smoke (ROS or what used to be called free radicals).
So - all of that is not good - directly causes inflammation and oxidative stress, AND - insulin resistance.
The net effect of this is that fructose acts as a catalyst to the otherwise slow process of insulin resistance building up over time, more insulin causing the "normal" to raise, and that requiring more insulin, becoming less sensitive to the action of insulin, requiring more... and so on. Sugar -table sugar- is of course both glucose and fructose which means that it has both effects at the same time, but having lots of fructose because you believe it to be natural - as I did - is a very bad idea.
The most obvious example of this in food industry is the production of Foi Gras. Literally meaning "fat goose" - it is the ground liver of geese specially fed to have fat livers - and how do they do this - well, it's no secret, you feed them high fructose corn syrup for the last couple of months of their life.
So - that's essentially why - as a teen (therefore never having had the 20 or so years or gradually increasing insulin resistance that typically leads to T2DM) you can still develop a fatty liver.
The good news, is (as you have shown) that your metabolism is so active at that age, that fairly small changes can have big, long-term effects. Those of us figuring this out in our later years have a tougher time of it.