When is sadness depression?

Been feeling really really utterly down and sad. I have not been out of the house much lately and when I do my anxiety is sky high and I yern to get home asap so much so that I have cancelled appointments and arrangements. Its not that home is all that great, I live alone, but the world just feels too much and I dont know what eles to do other then to stay in and try and contain my anxiety. I have no friends as I moved alot with my family and now I am more settled I feel like a huge wave of sadness and grief has crashed over me. I feel utterly isolated and like a waste of space in this world as all I do is exsist day to day. Other then breathing and sleeping and eating I exsist for nothing. I have a number of skills and talents and potentual in a number of creative and caring fields but I can barely think of anything other then how to wade through days and long dark nights where sadness drowns me. I want to be happy. I find joy in plenty of things but the joy is breif and takes a great deal of energy to sustain whilst holding back my grief and sorrow.

Has anyone any advice on how to move through this? Would a big change or challenge be the answer? I have a superb mask to the real world which makes me appear very happy and creative and capable but I am barely connected to that person. I cant even drop the mask to say and show how desperatly sad I am. The internal hurt is so intense Im stunned and frustrated by it and tired of it.

If this is sadness then what is depression? What can I do to become alive again? How can I stop the sadness?

Thanks