How can you really fix someone who’s too broken to be even fix??

Should i run away from him? I always think i could fix him somehow…but i think its just all in my head, always dreaming abt fixing him but in reality I couldn’t. His too full of his self—a cheater, might even be a narcissist (sometimes?), a personality that i cant even describe. But he kept saying he changed, and i think he did but not totally u know.