When Rey was first introduced and kept insisting her parents were coming back, you could tell pretty much everyone was thinking "Uh-huh. Riiiight." Even she seemed half-hearted about it and eventually gave it up. Once she's accepted that her parents weren't coming back, then it's easier to accept the other memory, that they left her with no intention of coming back and never will.
In short, denial. If she refused to accept they weren't coming back, there was no reason to accept the memory they were nobodies her sold her off and left her behind. Once she got over the first hump, then she could accept the latter memory.