You said your parents won't believe it, well, it sounds like it's true. Even if it turns out it isn't bipolar, there's a problem. I assume you're in high school, you can talk to the school counselors there free and in confidence (As long as you're not a harm to you or anyone else) I know my school, not sure about yours, offers group therapy. Maybe that will help?

You can also get your school counselor to help you convince your parents once you fill him/her in. It's considered neglect I'm pretty sure for them to deny you treatment you need.

In your parents defense, make sure you put yourself in their shoes. They may blame themselves, and it's easy to pretend it isn't your problem, easier then them blaming themselves. Not that it sounds like you hold a grudge against them for it, just wanted to make sure ya know. ^_^

Good luck whatever you do~!