Nearsightedness is the name of the condition: the state of being nearsighted. Your eyes work better up close than far away. A doctor may say myopia; everyday American English says nearsightedness. British English more often names the same problem short-sightedness. It is a noun, so you have nearsightedness, or you suffer from it. You are not 'a nearsightedness' yourself — that would mix the noun with the adjective. Glasses, contact lenses, or surgery are how people live with it. The word does not change the meaning of nearsighted; it only turns the describing word into the name of the thing.