I have buddies that served in Afghanistan and what I'll share will sound shitty, but reality sometimes is simply shitty.
The country is just full of uneducated superstitious, goat herders. There isn't real allegiance to "Afghanistan" as a country, so building a unified democracy is/was pretty much impossible.
They were training Afghan troops but would never turn their backs on them lol, they said they'd frequently try to pull shit and the concept of discipline, leadership that you need for a stable military / police simply isn't in the culture.
People were hoping for post ww2 Germany, but Germany was a country filled with educated people with a history of order Afghanistan has just been tribes at war for decades.... There was no way to make it work.
That's not to say that Afghanistan doesn't have good people in it that just want peace feed their family and live, but at its current state it's not going to be anything but a theocratic hell hole.