Why do I get agitated so easily?

Agitation occurs frequently for me. When I am agitated I get the feeling there is something building up in my chest and I have to stretch my shoulders back to release the feeling. Certain things agitate me; for example my mother clears her throat quietly a lot but every time she does I feel as though I have to imitate it so I don't get intensely irritated and bothered. Also, being in the presence of some people (such as my stepdad) makes me uncomfortable if I can hear them breathing or if they're close to me.

When I am at home I feel as though I am constantly in a mental malaise and as though there is something wrong in my life. Life in itself scares me and I am terrified of leaving school because the daily routine of going to school is familiar to me. I am terrified of dying and growing old because I think at that point my whole life will have been worthless and like nobody cared about me (and if they did it would have been meaningless anyway). I also get depressing thoughts a lot, however at school I am generally happy as I am occupied and in the presence of friends (although yesterday I cried for an hour at lunch without much reason when they were there which felt quite nice).

Sorry for going off on a tangent and writing a giant rambling paragraph. I'm not quite sure what I am asking but I'd like some answers anyway.

Thank you