I've never been in the situation, but I hope you find a way to get rid of her. I don't imagine this helps your anxiety much.
It's good you decided not to change your username.
My sister had an online & offline stalker before. I think eventually he backed off - police were never involved to my knowledge. She's not an internet addict like myself, but she still had to temporarily delete her facebook account and switch email addresses a few times. He would sign in as her and pretend to be her. It was really scary reading someone mimic her mannerisms and writing style so accurately.