Mechanically speaking, why not just start your ghost as her Darkest Self? That is pretty much what you have described anyway. Maybe once her sister or her boyfriend expresses regret at her death, or forgives her, she will be able to interact again. Or maybe she has to forgive herself for her suicide and the tragedy she has caused.
Run with it for a session or so to play up the angst, but then come back. The Ghost's Darkest Self is a great and completely appropriate gimmick, but I agree that over multiple sessions wouldn't be fun. In the meantime, go and spy on all of your friends and enemies, trying in vain to get them to notice you while you witness their secrets. Maybe someone you once knew has a touch of the spooky and can see ghosts? Bonus points if they are someone you were constantly at odds with, so now you have to win them over to your side before they will help you.
As far as the long term interaction with the town goes, maybe the supernatural "rules" of your setting could allow for ghosts to alter their appearance with their ectoplasm or something? (For better spookery, of course.) She can be the new girl in school who looks sooo familiar, yet nobody can quite put their finger on it despite the fact that she has the same first name as one of the girls who died in that awful thing last month. What was her last name again? Oh, I don't know. I have trouble remembering. Must not have been too special, then...