I don't think I've ever actually been properly tired to fall asleep in my whole adult life and I think that might be the entire problem
I'm going to bed every night at a reasonable time, because I know that's what I'm supposed to do. 10:30, maybe 11. And I lie down and I'm nowhere near tired. Not even wired or stressed or anything. Just... Totally neutral, like someone who has been told to go to sleep as a chore, not because their body is even remotely demanding unconsciousness.
I've done this for years and I always assumed it was normal. Some people can just roll over and fall asleep, others need a bit more time to drift off, and I was in the 'second camp,' just how my brain is built.
Then a couple of months ago, my work schedule totally imploded for three weeks straight, I was working from 8 AM until 1-2 AM. It was horrendous in a lot of ways, but there was one specific thing that I noticed; I actually felt tired when I finally got into bed. Like I was ready to be unconscious and fall asleep instantly. I've never fallen asleep so quickly before, and although I was getting fewer hours, I felt strangely more refreshed in the morning.
Once that phase of working until the early hours was over and I was back to my normal 10:30 PM bed time, it became impossible. I'd lie awake for an hour doing nothing but staring at the ceiling, just like always.
So I looked into it a little bit and it turns out that some people have naturally delayed sleep phases; basically, your body clock runs a bit later than what the normal human schedule expects. Ten thirty for some people like me, in essence, is like asking someone else to go to sleep at 8 pm, just because their body isn't biologically ready yet.
I haven't entirely worked out what to do with that information, since I still need to be at work at a set time, and I can't just decide to sleep from 2-9 pm every night. However, it is quietly comforting to know that all those years I was doing absolutely nothing wrong, my brain was just operating on a slightly later time scale and I had no idea it was what it was.