Eventually, I got slightly better at sitting still and "focusing". I would (and still do) fidget like crazy, either pen spinning, tapping my feet, rocking back and forth while "focusing". Focusing was just me quickly switching between what I was supposed to be doing and anything else, back and forth.
The upside is now I am basically "immune" to distractions, I can always reply to slack/my girlfriend/whatsapp messages/alerts/PR feedback/emails/questions and then go back to what I was doing 5 seconds ago without any issues. I've probably switched between this comment box and coding 20 times at this point.
I tried getting an adult diagnosis in the UK to see if maybe "proper drugs" would be helpful, but was just told politely to fuck off by the GP. The reasoning was that I have a job in tech, so even if I had ADHD, it's not bad enough to warrant any treatment. Lots of coffee and listening to "hardstyle" or any music with high BPM seems to help me focus.