I think I may be bipolar but I'm not sure?

I'm almost 15 and I was told about two years ago by one of my closest friends that he thought I might be bipolar and since then six of them have said so. I have a really high stress level, especially at home. My family has been on the brink of splitting up for almost three years now and my mom is stubbornly trying to fix it even though my dad is one of the biggest jack-donkeys I know.I sometimes seem to be so happy that people ask me to slow down when I talk so they can understand or they can't keep up with my topic changes and other times I'm so sad that I snap at the littlest things and I cry for no apparent reason. I usually need very little sleep but at times I feel like it is all I can do. My appetite is usually really big but at times it will almost diasappear completely. I self-harm whithout anyone knowing and sometimes I even contemplate killing mysef before I think of my little brother and sisters and what that would do to them. I have been trying to control my mood swings ever since I started looking into it, hoping that it wasn't bipolar or it was one of those cases that just needed self control but it has gotten worse and I didn't know how to deal with it anymore,it was scareing me. I told my parents and they put it off saying I was over-reactng. I don't know how they could think that since they saw me at one of my bad times hoding a knife to my wrists and when i couldn't I ran away before being caught. I can't talk about this to anyone I know so I'm hoping someone who sees this can help.