Why am i so Unhappy in my life? Please Help?

I'm always feeling down and have negative energy around me. I'm 17 years old and I've been failing in school and thinking about dropping out, which i won't. I am a very sensitive person, and I've been diagnosed with depression. I've been to therapy, and she didn't help me with anything but giving me attitude. It's stupid, i hate it, and i'm never going back there again. The reason I've been feeling down lately is because i'm very worried about my future, and it's making me not wanna do anything anymore but concentrate on that. I have my life planned out ahead of me apparently, but i'm not really sure it will turn out the way i want it to. I wanna become an actor when i grow up because that's the only thing that makes me happy, but i can't afford the acting classes. I remember when i was really happy last year, because i hadn't had a care in the world but just be happy and have fun. 4 months ago i wanted to kill myself, because of how depressed i was and my teacher knew about it and told my counselor, and i ended up being in group therapy, which it was a waste of time. I still don't know why i exist in this world, i am always insecure, have low self esteem, and not much of confidence left in me. I feel like i'm worthless. I just don't know what to do anymore... i literally feel like crying every night, i feel like i wanna give up on everything. I hate this life.