I need a way out

I'm 15 years old and for the past two years, coming and going, I've suffered from what appears to be something like clinical depression - but it's been pushed aside pretty well for the past year until now. Just over a year ago I witnessed my grandmother's death and since then it's been downhill with distraction on top, if that makes sense. My parents have split up and I've been finding it increasingly hard to keep up with schoolwork because of being unable to make myself ever do it - I just can't any more. My three closest and most important friends have all drifted away in different ways - one has started socialising with a new group through one of her interests in a certain series; I tried this out and joined in for a bit but I just can't fit and I'm not really included so I'm not going to force it. That group and that whole series and fanbase seems to be all she cares about right now. My other friend is older than me and has a lot going on, so I'm lucky if I see her every couple of months - and the lack of contact and updates from her, which I don't blame her for, is a big source of my feelings now. My other close friend has a lot of her own stress and some problems going on too, so again I don't blame her but she's kind of distancing as well. I just finished the second to last year of secondary school so I had a lot going on to distract me but now it's the holidays and I've got a bit less to do I've had to come to terms with my mental state - and I'm getting desperate. I did see someone about a week ago for a chat to figure out how I am and what step to take, but we didn't talk about very much of what was really getting me down - she's a lovely woman but talking to her doesn't help me. I'm getting desperate and every small thing that brings a little stress or responsibility pushes me over the edge; I feel like I'm going insane because I'm thinking some pretty twisted things and my only relief and happiness comes from thinking of new and frankly brute or disgusting ways to kill myself - though I know I wouldn't have the nerve to do it. In all honesty I've started to envy those in asylums - I find myself having outbursts in which I will sit crying or laughing, sometimes at the same time, while thinking of my own suicide or the state my mind is in due to stress. At one point a few weeks ago I started seeing things, thinking I wasn't alone at any time and that I was seeing figures out of the corners of my eyes. To be honest I'm a bit of a child; if I don't get attention or comfort for some time (in other words a couple of days, tops) I start to feel alone and distanced. I have only spoken to my friends about this and I want to avoid, more than anything else, my parents finding out; I don't want them blaming themselves or treating me differently and I'd feel very awkward. But I wish I could be taken out of life for a bit, because I go back to school soon and I just can't take it any more. Hence the envy of patients in psychiatric units... I feel like I need some time to get my mind back on track, totally out of the stressful loop of life I'm having to be stuck in now, because I think if I'm dropped into this loop again I will crack totally. I'm sorry to post all this here suddenly but I'm unsure of what I can do now and I really, really need some answers.