Alone

Sometimes I feel like the last couple years have been a downhill slope to complete breakdown.
It doesn't make any sense. I shouldn't be like this.
I started high school in 2014, with mostly online classes and an orchestra class at the public high school. Despite my initial anxiety (I'd always been home-schooled before that) I did really well, passing both semesters with straight A's. I started being a little more social and outgoing.
There were low points. I began seeing a therapist around that time for depression and social anxiety. Talking it out seemed to help. I also had suicidal thoughts, but I could always link my bad days to hormones. As I said, the therapy helped some.
This school year started out great. I got into Chamber Orchestra (best in the school; I had to audition for it). I started playing viola along with my violin, piano, and pipe organ. I got another semester of A's. I'm planning on running start. I'm looking for a job so I can buy a car when I turn sixteen and get my license.
But now I'm self-harming regularly and I don't want to talk about it with my therapist. I told a couple friends, but only about my previous scratching and choking. No one knows about my cutting, which started recently.
Last summer, my maternal grandmother started losing her battle with cancer and other health issues. We've been close for the last couple years; she's my piano and organ teacher. She's on hospice care now. I don't see her much; I know I'm being horribly selfish but it hurts too much.
I feel so weird right now. Like I'm detached from everything. I used to really care about my schoolwork, now not so much. I used to care about my musical instruments, now I don't. I never want to think about myself. The only thing that makes me happy, if just for a little while, is reading and writing fan fiction. I do that a lot because it keeps my mind off my life.
The first time I cut, I was trying to punish myself for hurting my mother's feelings. Now I do it because it gets rid of the knot that feels like it's always in my stomach now. I can feel it while I write, and I know that before I go to bed I'll get out a knife and cut myself again.
I got desperate yesterday when I finished orchestra class. I had to go to church to practice the organ, but there was no one there so I looked around for something to cut myself with. I don't carry a sharp knife with me unless I'm taking a walk and that particular knife is for self-defense only, not self-harm. In the end I had to use a table knife. I literally had to saw at my skin to get any blood. Then I had to cut myself at home later anyway.
I feel trapped and alone at the same time. In front of everyone else I'm smiling and laughing and joking. I'm acting like I'm on top of the world, like I'm maturing well and going to go on and be successful in life. But inwardly I feel like I'm a horrible excuse for a person. 
I've told my mother that I don't need to see a therapist anymore because I'm fine. Really I just don't want anyone to find out what I'm doing to myself...I can't tell my best friend because I'm afraid he'll be ashamed of me. I can't get a job at the swimming pool because when I'm in a swimsuit all my cuts are visible. I won't be able to wear short-sleeved shirts as long as I'm cutting. My parents will be so disappointed in me when they realize what I'm turning into. Everyone will be.
So I'm alone. And I don't know how to be any other way else anymore.