Other people have probably written similar stuff to this, but I have a sister who's an extravert but had a time when she was very insecure. (She's a lot better now! :D thank goodness)
From what I remember, she took everything (and I mean everything) personally-- someone makes a joke about someone else and she'd think they're talking about her, and she'd take the slightest bit of information given to her and think the tone of the person saying it was insinuating some kind of criticism towards her.
She would have super high highs when with other people and then super low lows when she was by herself because her self-acceptance came from other people's positive reactions, and she would worry over any non-positive (even indifferent) reactions she had previously received.
She would also get really defensive about the smallest things even when they were not said to her to give offense, BUT this would only be towards people like family and very, very close friends-- in front of the other people whom she seeks general acceptance, she would smile and play it off, then stew over it in private. She was also really, really bad at saying 'no' because she was afraid other people might reject her if she wasn't all smiles and 'yes'es all the time. She would also take most positive comments about her as just flattery but take all negative comments to heart.
These might be a good starting point? Haha as an introvert myself, I had so much about her that I didn't understand when she was feeling insecure, so I remember a lot. Hope that helps, lol