If you change your password on your PC then it will not be saved at any other locations at which you might have used it. I know this from personal experience.
And it's not at all childish to involve your parents if safety is involved. I'm sure your parents know of the situation. You wouldn't even have to lie about who it is you're trying to avoid. Parents, no matter what your age, are always there for you and can be surprisingly understanding. After my accident a few months ago they took me home for a week to care for me. You just do what you gotta do. It's better to have fears of looking like a dork (which you don't) than to fear for your life. I so feel for you and I hope that you take some of the advice (mine and others') that you get. We are all here for you.