  
(or other close childhood relatives)

Because they were actually there when you grew up, had the same parents, grew up in the same household. So if they say your experiences aren't true, they must be right, right? Then it must be me who's the problem, who's exaggerating, who's in the wrong. 

This is still the hardest part in my recovery because as I'm trying to learn  to validate my own feelings/experiences, these people are constantly there to remind me I'm the one making a problem out of everything. They are not even bad people, it's like they have the luxury of not being able to imagine feeling so awful all your life, that for them it can't be true.