Is this depression or something else?

I'll go through days or sometimes not even that, sometimes hours of feeling seriously down. I'll cry for hours and not be able to stop or I'll have all of these negative thoughts running through my head and but I can't express them. Whichever way my feelings manifest themselves I feel completely trapped. Sometimes I just want to talk to someone but feel there's no one there to talk to. Everyone has things of their own to concentrate on, I don't want to push people away because they realise what I can be like. Other times I will talk and it may help but then the talking will end and I'm back to square one. Other times I have no idea what I would say which makes me feel even more alone because it's almost like if I can't express it properly it's not there, but I know it is. Often I want to hurt myself because I feel that somehow that would help. I don't think about it, it's more of an instinctive thing that I stop myself from doing. Sometimes the idea of death is wonderful when I can't think clearly about the consequences of it. Other times the thought of dying makes me feel even more down, and hopeless. As if death is an inevitable that I'm terrified of, and completely dread. What is this? It's not constant sometimes I'm fine but when I'm in this state It's unbearable and I just want to know what it is and how I can overcome it.