Damn. Why do I feel bad for her now.
I'd say it's because it was made very clear that she thought this way not (only) because of her own presumptions, but that Literally Everyone said to her growing up "You're gonna marry the Prince so shape up, girl!" So she did just that. She played her part, tried to be as perfect as necessary for the guy she genuinely seemed to love and would have a chance to show that love and prove thag she was worth it, even if he didn't think anything of her at the time. The heart can change, after all, right? This was a since childhood thing that she'd been subjected to.
Then ALSO literally out of absolutely nowhere she gets the news 'You're not marrying the Prince, some second string princess from a snobby kingdom is, probably for political reasons.' So of course she gets pissed thinking some Green Tea Bitch is walking in and stealing 'her man'. Again, something that had been all but promised to her by her family and powerful nobles for years and years. Suddenly, snatched from her in moments. I don't blame her for going 'wait what the fuck is this bitch doing here!?' And she simply doesn't want her years and years of effort and love to be tossed aside.
But, the heart DOES want what it wants, oftentimes. And his never wanted her. She had hoped he would eventually change his mind, whether pre or post marriage, but it never did. The idea that he never did or would love her like she loved him shattered her psychologically because it basically confirms her worst fears. All the crimping, and fighting to stay on top, and the court intrigue, and all this horseshit she has had to go through for the sake of her happy ending has been for Absolutely, Totally Nothing, You Have Wasted Your Life Completely Up To This Point, Good Day Ma'am.
I'm ultimately not saying she's in the right, she's not and deserves to at least be jailed, but in a way she's a product of her environment and was ultimately someone blinded by love to the truth, which is all too common. I don't forgive her, but I do pity her and get WHY she did it all.