Dead family member. Dead love. First love. Past love. Things that she can't let go of that causes problems in her relationship with the main guy. If they aren't dead, maybe the past love comes back and that causes even more problems like lingering feelings.

Some examples:

Yume de Aetera - past love/first love

Nagisa's first love, Minato comes back into her life and while she's in love with Fuguno, she almosts marries Minato.

Maison Ikkoku

Kyoko falls in love with Godai pretty early on, but she's hung up on her dead husband for a good bit of the story. Even when she's with Godai, it shows her feelings for her dead love still linger as Godai finds her crying over his items and the kicker is... IT'S A WEEK BEFORE THEIR WEDDING.