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Memento's backwards narrative can feel somewhat gimmicky. but it forces us to live through Leonard's perception of the world, seeing a scene with no knowledge of the (usually critical) events that lead up to it.
At the end of the movie, you can be left with two conclusions: that Teddy was lying to Leonard, or that he was telling the truth. Which means that either Teddy is a Manipulative Bastard, or that Leonard is taking advantage of himself just as much as the rest of the cast. Neither interpretation is especially happy, but which one is worse?