Undecided Question

I'm 17 and male.
I think i have some kind of anxiety disorder and i also think i have Depression.
I feel really low and completly uninterested with life. I'm in a stupid course at 6th form school because i couldn't think of anything else to do. I don't have a clue what i'm going to do in terms of a job, I don't have any aspirations in life and no real interests. I don't really care about anything at all. I
play on my Xbox all day because it's the quickest way to get through the day.
I'm quite a private person and i have a huge problem with trusting people. Everyone is out there for themselves and will stab you in the back to get what they want. I just can't trust anybody, not even my family. Every time someone leaves the room i always have thoughts about what they're doing. most of the time i manage to distract myself from these thoughts but the point is i have them.
I'm a bit of a hermit and i stay in my house most of the time the only time i leave my house is when i go to school or when i go to my dad's. I like to keep a low profile in places that i feel uncomfortable in (which happens to be almost everywhere). I don't hang around with my mates, because i don't even trust THEM!!!!!
When i do decide to go out of my house, i have really horrible thoughts. Basically they're "what if" thoughts and the bad thing is they're always negative thoughts.
For example. I was walking near my house and i saw a truck and thought "what if that truck blew up". I then got horrible images of it exploding and bits of it flying everywhere and people getting killed. This happens to me when i'm indoors, but most of the time it happens outdoors. Sometimes when i walk past someone i get it into my head that they are going to do something and i get a bit freaked out. To take my mind off these negative thoughts i count random things like buildings or i add up the numbers on car licence plates. When i'm outside i try my hardest to avoid any cracks in the pavement. I'm quite claustrophobic and i hate people touching me. I'm also really impatient and i'm very easily irritable. I'm a very anxious person constantly panic and get worked up about things. I get really nervous sometimes and i bite my lip and chew my fingers. I have bitten my lip until it's bled multiple times, i've also bitten my fingers until they've bled. Once i was so nervous and anxious that i got my arm and i scrapped it with my teeth until i realised i'd broke the skin.
When i'm trying to get to sleep, i usually get scary thoughts about dying and about scary creatures and i get so freaked out. I then distract myself from these thoughts by sorting things out ,like my DVDs, and put them in order or make them symmetrical or alphabetise them.

I feel like i'm about to just cave in and have a huge breakdown.
I don't even know when all this started.
I'm not really an emotional person, and i try not to share my feelings with others, but i just needed to share them with someone and i'd prefer it to be someone i don't know, rather than my parents or my doctor. That's why i've written all this,

So…
What do you think.
Why am i so messed up at 17?