I think it goes back further than that, sure, Oliver was the "prince" in her mind and she thought they were going to get married and have a picture-perfect life but she seems to have been holding to this rigid idea of "must be married by 30 and have kids" even before they got together - from childhood probably, Disney movies and seeing her parents' and Tom and Gi's relationshipsShe clearly cannot let go of the life with Oliver that she had planned out in her head. She wanted him to propose and for them to get married, so she’s just done it with the first person who came along.
Althhough it still boggles my mind as to how were JoJo and his bizarre hairline moving in with her on the first date? I seriously think, either on the date itself or when he went sliding into her DMs, that they had a discussion and laid out expectations and agreed to get married very quickly. There's nothing wrong with that, but it's essentially a business transaction, not the whirlwind fairytale romance they want to pretend they have