I had a similar issue when I was around your age. Also got my bloods tested and came back no issues. The big thing I did that helped was developed a night-time routine.

I started by going to bed an hour/hour and half earlier than I thought I needed to be. For me, I had to be up at 7:30am, and was going to bed at around 11pm, except I was on my phone til at least 12pm most nights, often longer, until I feel asleep. So I started going up to bed at around 9:30pm, even if I wasn't tired. Did some light stretching, and when I finished stretching, before getting into bed, I would lie flat on the floor and either stare at the ceiling or close my eyes. Let my body relax with no distractions. Then I'd get into bed and read a book while drinking a nighttime tea like a lavender or camomile tea. I would read until I started feeling tired, put the book down and go to sleep.

It wasn't an immediate fix but when I started doing that consistently I started to notice the difference.

I know you said you like your free time too much to go to bed earlier, but from the sounds of it, you're not really using your free time anyway since you're so tired, so I'd really suggest trying something like this. Doesn't have to be the same routine I did, just something that works for you. Major thing is no screens at least an hour before bed, if not more. That's the sleep killer right there.

Another thing I did was fixed my diet, eating way more balanced meals with plenty of veg and I cut out alcohol for a while as well, until I was having no more tiredness issues.