Feeling really depressed?!!?

I have been self harming for about 2-3 months now, right now my left arm is covered in cuts and a few scars I cut on my leg earlier because my arm felt soo sore, I've cut words like love, *****, mess, and fk up into my arms. I do it because I feel depressed a lot of the time and it makes me feel better and I feel more in control when I'm stressed or angry. But the thing is I dont know WHY I feel depressed most of the time, I feel like I'm useless hated like no body cares I hate myself, I just feel like curling up in a corner and dying. One week I can feel fine but the next I can feel like throwing myself off a bridge. I've talked to a doctor about my SH and I've Been referred to some place, and I'm waiting for a call. I've talked to my science teacher and pe teachet about my sh, my science teacher knows I've had suicidal thoughts, I want to go to him or my pe teacher to talk about me feeling depressed all the time, but I don't know what to say. And whenever he asks me if I'm okay I just say I'm fine. What do u think I shud say to them? What shud I do? Please help? X ty