I've learnt to appreciate all the gloomy, sad happenings in life, because without them, the uplifting, happy things could not exist.

For some reason, most people would sit down and watch a sad movie that makes them cry, but still be glad that they watched it and enjoyed the experience, that is proof that even in the depressing, gloomy, parts of life, some sort of enjoyment can still be found.

Because he was unaware of his condition, it means he lived with more to gain in life and with more to gain comes more to live for, thus, you can be sure he lived life the way it is supposed to be lived.

Looking back to when I was undiagnosed, while I may have been worse off, life still had it's joyful and sorrowful moments as usual despite my ignorance.

Ultimately, you should celebrate your dad's life for the fact that the only reason it's possible for you to understand and know about your ADHD is because it's also possible to be unaware of one's own ADHD like your dad was. Experiencing the bad things in life is still a privilege, because those bad things mean good things also exist.