They both ignored each others. The fl because she was afraid of ml being a big burly man who never smiled and much more older than her. And ml because he didn't know what to do with a CHILD who resented him so much. He thought the only thing that could make her happy was luxurious things, so he gave her whatever she desired, WHILE not thinking that the fl WAS a child, a 9 years old child, who didn't even understand what marriage is, being separated from her family in a short time, and not long after her nanny passed away, leaving her ALL ALONE in a place where she didn't know anyone.
The ml know fl was having a hard time, thus why she holed herself up in her room all day. He could have reached out to her when her nanny passed away, but he didn't, because he didn't know how. And her, being lonely, it was easy for her to take a hand that was reached her even though in the form of cheating. She needed someone to make her feel less lonely. That didn't make her cheating right, but that didn't mean we couldn't understand her reason.
If you said the problem is her personality, just remember in her first life she was REALLY an 18 years old girl (in the timeline this story happened) who didn't have anyone to teach her about anything and was being left alone for YEARS. And in this 2nd life of her, her mind is already MUCH older, and already went through so much life lesson, thus why she gets better in handling her emotion.
She just need a proper guidance from the start, which no one gave her.
There's no children in life who could know which one is wrong and which one is right without anyone teaching them.