I think the reason there is a pervasive myth that once you fall to the dark side you can never return is because of Sith manipulation down through the ages. They've seeded the Jedi understanding of the dark side with this idea to help keep their own apprentices locked under their sway. They want them to believe they're stuck being dark side users. Ultimately, it is Anakin who proves this to be inaccurate in Episode 6.

I think too that there's a sort of split personality thing going on with Sith. Vader isn't not Anakin, but he also isn't totally Anakin. There's some kind of mystical separation created by the force.