How do you repair broken heat damaged hair?

My hair used to be really nice and straight and lovely and silky, then for some stupid reason i started straightening it when it didnt even need straightening, now i have to straighten my hair to get it straight because now it goes wavy. But it doesnt always stay straight when i straighten it now because its heat damadged and broken !! whereas my hair used to be naturally straight!! i miss my naturally straight hair what do i do to get it back to how it was???? someone please help!