What causes my behaviour is that my brain is literally retarded
That is a very infectious way of modelling the world.
Asking "what is causing it?" is the dead end.
It certainly is not. If I have problems geting the information from a book in to my brain. The cause can be said to be that I am trying to read it. Solution: Get it as an audiobook instead.
I do have an ADHD diagnosis but I don't think that justifies my thoughts in any way.
> The cause can be said to be that I am trying to read it.
No it can't, because most other people can get the information from reading it just fine. The cause is that you're reading it with ADHD.
But we're talking at odds here - I'm not talking about finding coping strategies, but looking for the root cause of ADHD. For the current state of medical science, the latter doesn't really help with the former.
I do have an ADHD diagnosis but I don't think that justifies my thoughts in any way.