This is because Elly is a short-sighted idiot who can't remember the past all that well. Not only can't she remember that she's proven herself not to mean what she says and that she can't stick to deals she makes because of her self-centered inability to see anything anyone else does as good enough, she also doesn't realize that her family has started tuning out her endless beefing about how horrible her life actually isn't. Since she's an unreliable, inconsistent blowhard who never seems to understand what's happening even (or especially) when it's right in front of her face, all her children can remember is that Mommy hates them and doesn't think anything they do can ever be right. Since they can never win, they don't even try.
As for remembering things that she wishes that they had forgotten, what Elly forgets is that her children are as prone to collecting injustices as she is. She feels justified in keeping track of slights and indignities that are decades old but when her children do the same thing to her that she does to everyone else, they are in the wrong and must learn to let things go.