Is it possible for someone to actually learn something and be a productive member of society if they have brain damage or some sort of moderate to severe mental disability?

I feel like I'm pretty retarded, I've had several concussions as a child and other problems like chronic sleep deprivation which I think has affected my mental development. Can someone shine a bit of light as to how this manifests in adults, are they just duller and have worst memory or whatever so require more work or do they simply not have the capacity for good learning? Can a "retard" just read a bunch of books and study a lot and actually come up with fairly novel concepts on some topics, not necessarily become a professor or genius.