Panel 1: Anne appears to be trying to process the situation she finds herself in by the only way that makes sense to her when things go wrong in life by blaming herself. This is why she asks Elly what is wrong with her as a person that would make Steve see another person.
We could probably skip the next two panels to get to Elly's inevitable, literal-minded, "I look like a woman your age but my mind stopped developing when I turned five" reaction but we don't.
Panel 2: What we do get is Anne asking if it's because she's not beautiful and/or doesn't have a glamorous career that Steve has strayed because it doesn't appear to occur to her to come to the conclusion Elly wants her to.
This, I think, is because Lynn doesn't get irony. Anne could mean something that isn't "I'm defective" but do so in a manner that goes so far above Elly's head, it doesn't even muss her hair.
Panel 3: My suspicion that Anne isn't actually letting Steve off the hook, never has and never will starts to gel into something like confirmation when she concludes her want with "Is it because I'm a wife and mother who's spent the last ten years making a home for him? What's wrong with me?"
This is because I'm not a child like Lynn and can understand that the unspoken answer is "Not a fucking thing.....Steve is just a fucking fool who doesn't know a good thing when he's got it."
Panel 4: Since Elly is a well-meaning infant and doesn't quite get that the question Anne asked can mean something else, she consoles Anne while thought-bubbling "Why is it that when one partner is cheating, the other asks "What's wrong with me?""
Summary: As I keep saying, Anne COULD mean "Hey.....I'm his wife and he should be satisfied with that so what the fuck is wrong with HIM?!" but since Elly and Lynn are literal-minded nitwits, we eventually end up at a Tim Horton's watching the North American Turnip-Nosed Jabbering Dolt continue to not understand the person she wrote off as a weakling.