Sorry if this has been posted or if it's just a stupid question, and maybe this is because I don't really understand how short term memory loss works, but how can Leonard remember he has a 'condition' to begin with?
No, it's not a stupid question, although the "default" reason that the movie's detractors give as to why the movie is one huge plot hole.
As I see it, the effects of brain damage aren't black and white, especially since Leonard has his amnesia from trauma. There are only two or three real-life examples of human patients with anterograde amnesia, and I know that two of them basically had their entire hippocampus taken out. H.M. (the most famous) was apparently aware he had memory impairment, but Clive Weaving does not know. He does somewhat remember very emotional events in his life after his illness, like his brother dying. He gets sad when someone mentions his brother but he doesn't know why. Weaving's memory is also extremely bad - he only remembers 7-30 seconds at a time. He still remembers how to play piano and conduct even though he doesn't remember that he went to school for it. He even forgets that he was making music at all right after the piece ends and he basically goes into fits for several minutes afterwards. The brain is a really strange thing.
So...yeah. Realistically, it depends on how Leonard's brain was affected by the injury and how much it managed to recover.