But she has been consistently humiliated and generally treated like shit
That might have happened less if she hadn't proactively treated everyone else like shit. She has treated everyone in Ambridge like dirt from the first meeting, irrespective of who they are and including people who have tried to welcome her/offer her hospitality.
When she was snarking at Lilian's decor in the Dower House the first time I thought it was before any relationship with Justin had started (or at least before she knew anything about it)? It was just gratuitous nastiness.
She made it pretty clear to Lilian early on that she had as many affairs on the side as Justin and was quite happy to use unimportant people as disposable entertainment.
She and Justin deserve each other frankly.