I think it was more of a naiveness. We really can't blame the past FL. Her father favors her cousin more and her fiance was cold to her. She didn't know at that time that he loved her because she wasn't aware of his thoughts unlike us the readers. To her pov, her fiance must have appeared to not want the engagement. Like if you have a fiance who's cold too I don't think the engagement would last. Also it appears that the engagement was made for political reasons and not love. Of course, I can't say much since it currently has only 5 chapters.
And it seems that she doesn't receive much affection from her fiance and her father so she latches on to the first person that gave it to her which is Reagan. Again, we really can't blame her that much because all she wants is to be loved. If in our pov as readers, we can see the thoughts of each character thus we can see the trashy reagan, trashta wannabe cousin, her probably pos father and the sweet but cold ml. To her past pov, a lover, her sweet cousin, her father and her cold fiance.
That's my thoughts so I can't wait for the future chapters โก(> เจ <)โก
The ML (second prince) is so kind, the world doesn't deserve him. I do understand the FL a bit since in the past the Second Prince is acting cold towards her but still it doesn't justify her cheating on him with his brother. :<
ANYWAYS, this Vienna sausage and the Fifth prince are the new Sovieshit and Trashta ugh