I'm still baffled by it, but a couple of years ago I found a switch in my brain and now I rarely have issues falling asleep anymore. I think the most important thing for me is not to constantly ask myself why I'm failing at such a seemingly simple task, but rather work around my natural instincts and try to support them as best I can.

If I go to bed early, because I have to get up early, I start spiralling and fall asleep way later, so I try to only go to bed whenever I'm tired. If I end up sleping too little I take one or two 20 minute naps (I try to never exceed 20, it would make me even more tired).

Another factor is probably how long I sleep. I always assumed I wanted to sleep long, because getting up sucks. But at some point I realised, that my first 'sorta-awake' moment is the perfect moment to get up and have energy for the day and get tired at the proper time. This moment comes at around 6 hours for me. I still struggle with this a lot though.