How could I leave my adopted family?

I lived them for a few years now and I wanna leave. They dont beat me or hurt me in anyway, they just dont love me... They have 3 other kids, all of who are their real kids, and they seem to love them more. They never really talk to me and never get me anything. On my birthday they just bought me a cake while on my other siblings got a bunch of toys and a much bigger cake. When it come to talking with me they never really seem to try and Im always their last choice for everything. Its really saddening and I cry almost everynight because of this. My parents know about me crying but have tried very little to comfort me. I wanna leave this place since Im basically not really family and Im not loved. Is there a way for me to leave and live on my own or go to another family? Im only 15. I dont think I could wait another 3 years.