What teaches us what drives this need of hers to stiff-arm catastrophe is less a default negativity that makes her see the world as something akin to a side-scroller in which everything is out to crush her and more a need to stave off something personal is the occasional appearance of her mother in the strip. When Marian bounces into view grinning her vapid grin so full of hate it hurts, we know that a woman who strives to project an aura of competence and authority is going to be treated like an incompetent and defiant child by an older woman whose need to be needed overrides her sense of propriety. Marian doesn't see the damage she's doing because she can't abide a world where she's the one who's waited on; just as Jim had to tinker because stopping means death, Marian has to take over the parenting because if she rests even for a second, she'll drop dead on the spot. If Elly's feelings get hurt and she ends up being humiliated in front her of family, well, she'll get over it and life will somehow work out for her BUT the point is that Marian must not keel over and die.
Eventually, of course, Marian realizes that maybe this need of hers to be the mother forever is actually a problem after all because what few photos Elly allows to be taken remind herself of all the times her bitter failure of a father made her feel like crap; Elly seems to have the same haunted expression on her face that she used to and it dawns on her that just maybe, Jim was right all along about how stupid it was to try to suppress Elly for her own good. Not, of course, that Marian ever explained why or apologized or anything. After all, she never wanted to look weak in front of people and admitting to being a colossal screw-up because she let her fear do her thinking for her is going to be a sort of humiliation from which she could never really recover. Best to let Elly wonder why the mother who loved her so much, she HAD to sit on her never cared for her at all.