Is it that you're finding it difficult to decide to lay down and call it a day? Is it that you lay down, but can't fall sleep? Is it that you do fall sleep, but you don't feel rested on waking up? Is it that you're waking multiple times during the night and can't get into deep sleep? Do you feel physically or mentally tired? etc.
Another weird thing is that I feel sleepier during the day than I do at night.
1) You're being blown about by discursive thought
2) Your circadian rhythm is out of sync
For 1), I'd recommend taking up mindfulness meditation and practicing before bed. This will reduce stress and out you in a better mental state for sleeping.
2) could be caused by any number of things ranging from drugs (caffeine, alcohol, cannabis, sleep aids, etc.), insufficient exercise, not enough light in the morning, too much light in the evening, shifting bedtimes, noisy sleep environment, snoring, etc. Most of the comments around here will help with this problem though I'd be skeptical of anyone telling you to use medication or drugs for it. (I'm not anti-drug, I just think we don't understnad sleep or the brain well enough yet to effectively improve or induce sleep with drugs yet and most existing treatments seem to make you unconscious, but not necessarily asleep).
So I made two lists. One for projects, one for worries.
The list of all the projects was for things I wanted to do. Things like learn piano, visit Moscow, floss more regularly, literally every project. I saved that on dropbox. It had over 60 or 70 projects on it. I added 50 on the first day, then as I thought of more things or saw the visual triggers (that cookbook I bought to learn to cook more meals, the light that doesn't work because it has a weird bulb), I would add them to the list immediately over the next few weeks.
And another list of all the things I was worried about. Might be things like the client is 3 days late on payment, does he have the money? Did I make a fool of myself at X's birthday? Why does Z keep shooting my ideas down so savagely. I put these all in a temporary tab in notepad++, mixed in with other tabs I tend to have open. It persists between turning the computer on and off, but if you close the tab without saving, it just gets deleted. As it was temporary, I felt I could put embarrassing or even mean thoughts on it. I think this had 7 or 8 things.
With the projects I soon realised I couldn't do 60 things at once and focused on 4. The list is there to find more projects to do as I finish stuff.
With the worries, next time I look at it, most of them are gone. And I just close it, gone forever. First time I closed it, it was so liberating. After doing that every now and again over a month or two, I only have to do it once every blue moon, it's a useful trick when I start feeling a bit overwhelmed.
Might not work for you, but worth a shot.
This is called "worry time" and it has helped me in the past.
Another article describing it is here: https://healthypsych.com/psychology-tools-schedule-worry-tim...
It's not a silver bullet but it's a useful tool to have in one's toolbox.
I've had similar issues before and I've noticed that if I think about work, projects, my day, etc. I could go on for a long time without falling asleep. I began to notice that if I think about generally fiction/stories, I can fall asleep much more quickly. In my case I think about plots of movies I've recently watched (or a long time ago), whether it's Avengers or Dragon Ball etc. Perhaps you could give that a try.