It sounds to me as if she wasn't prepared to face the future with the ex-wife and MIL-to-be so much in the picture. I wonder if this was a mutual decision or more Scarlet getting fed up and saying enough. Louis is so clingy I wouldn't think he'd precipitate a break himself. He was hoping for mommy #3.
I also wonder how much she had stars in her eyes about the prestige of being a member of a royal family...it sure didn't take her long to launch a fashion line, which was a bit surprising for a lawyer...and then the reality of what it would actually entail, coupled with the dysfunction that is MT and Tessy, really hit her.