Elly only thinks that she's a centipede.
It stands to reason that the worst-case scenario for Elly when it comes to dealing with the threat That Girl represents is it becoming impossible for her to deny that she has misjudged Martha. It's akin to how April freaked out at Jim because she mistakenly believed that he wanted her to admit that she was this bad person who could only feel jealousy and envy when it became obvious that most of why she hates Becky is that she brings out the worst in her. If Elly admits that there's just something about Martha that she can't stand, she thinks that this means that she'd be paralyzed with doubt and confusion because she is thus magically this unfair person who is bad to people and not this immature person who married and had kids before she was ready.
Just as April dreaded being told "So what if you don't like or trust Becky for a dumb reason? That doesn't mean that envy is all you can feel" because it makes her an average person who just can't stand another average person and not the Best at being THE WORST, Elly lives in fear of being told "So what?" when it becomes obvious to her that she was way off-base about Martha. Since framing one's response to her panicky yapping about being a bad person that way makes her an average human being who let a bad first impression mess up her life instead of this superwoman people are supposed to rely on, she just can't deal with being like a lot of people who make big decisions in a hurry.
Just as April dreaded being told "So what if you don't like or trust Becky for a dumb reason? That doesn't mean that envy is all you can feel" because it makes her an average person who just can't stand another average person and not the Best at being THE WORST, Elly lives in fear of being told "So what?" when it becomes obvious to her that she was way off-base about Martha. Since framing one's response to her panicky yapping about being a bad person that way makes her an average human being who let a bad first impression mess up her life instead of this superwoman people are supposed to rely on, she just can't deal with being like a lot of people who make big decisions in a hurry.