It’s pretty much inconceivable that someone’s mind could just be rolled back 20 years like a video recording, but that doesn’t necessarily mean a patient or observer who describes it that way is lying exactly.
Any time you pull chronological narratives from your memory, you are basically reconstructing that narrative from miscellaneous associations (“I was on the red couch when I heard that, so it must have been at the Pine Street apartment before Kiki moved out” etc). If your memory is messed up, and half those landmarks are missing or cross-wired, you have to do the best you can, and saying “I don’t remember anything after 1998” might just be a sort of workable fiction. You know it’s not exactly right, but it explains the rough scale of the problem in a way you and others can work with. It tells people you’re not a fully functional adult, and that you don’t remember stuff to the level of detail they would expect if if happened in the last decade or two. So, more of a shorthand than a lie.
