friends??

Over the last few weeks I have been feeling left out and I feel that my friends don't really like me. On the weekend one of my friends had a party, that everyone wa invited to, but then she had a sleepover afterwards and only invited 10 out of 15 of our friends. I wasn't invited and the people that were invited to sleep over all met up earlier, got to the party together, had inside jokes from that day etc. 

I had been slightly depressed for a while but the feeling of not being included pushed me over and I've started cutting again and feeling like I'm worthless. 

I really don't know what to do or what to say to anyone. 

My parents (I'm 15 so they know most things) don't know that I cut, and they refuse to get me anti depressants. 

My 'best friend' has also started being really bitchy towards me( not in a joking way) and I honestly feel like everyone would be better off if I wasn't here. 

I don't know what I should do. Any advice?