why does itunes update and lose all information on the music files i have imported from cd's and the like and only remember what i have bought from itunes?

Recently when i have updated itunes to newer versions, when booted up itunes only remembers it has albums from the ones i have bought from itunes and not the many i have uploaded, i have found i can just tell it to import from the itunes file but this creates the problem that i have douplicates this is a mess and i am REALLY struggling to see how something like this could happen, you would not find this from windows media player

does anyone have a solution to this problem?