To start with, Liz is somewhat passive and tends to react to stress by curling up into the fetal position. While she might tell herself that she's a strong person who's in control of her life, history teaches us that she simply drifts through life subconsciously expecting to be rescued from anything that might possibly scare her or expose her to anxiety or even to having to make a decision for herself.
This is because Liz hasn't got any sort of courage in her. Much as Elly wasted her life being afraid of humiliation and failure, Liz tends to be incapacitated by most situations she encounters and can only do things when she wills herself into a comforting unawareness of her surroundings. Sadly, reality can only be kept at bay for only so long. This leads us to the last major personality flaw.
Said defect is her terror of being caught out and punished. From the time she tried to yell "I LITTLE!!!!" in an attempt to not accept the consequences that were only to be suffered by ugly brothers and sisters who breathe her air with her permission to her calling the Evil Career Woman an ice machine because the poor fool tried to get someone who thinks she deserves a free pass to be a jerk because everyone haaaaaaaaaaaaaaaaates her, Liz tries very hard to not see that she is mostly to blame for her problems.
Good thing for her that Anthony doesn't actually see her for who she is. Since he wants a passive idiot who will do any stupid thing in a doomed effort to please her parents, she's found the douchenozzle she deserves to marry for being an imbecile.