You are not Joe, trust me. Yes, you are struggling and encountering difficulties, but you're twice (if not more) the age they have and you've been through a lot. Don't put yourself in their shoes as it's not a fair comparison. After a certain age, you need twice as much time and effort to get half the result, unfortunately.

At their age, their brain should be at the top and their energy level should also be on the top. Not to mention their only goal should be studying, nothing more.

It is funny though, because writing is the opposite. Once I start writing, I get in a zone and rarely get distracted.

See? That's great! It's huge that you can do this in the afternoon and thus not only fill your day with something useful and productive, but also enjoy it.