Why is my cat depressed?

I got this cat since it was a kitten. It was always in trouble and looking for someone to play with. She never grew up with another cat and never seen another cat before. It always wanted to play and fight and she is now about 8 months old. Recently I got a new kitten and she is very playful just like my other cat. 2 weeks later she started acting depressed never wanted to play and always just sits in a corner and stares. The new kitten always tries to play with her and the other cat always hisses at the kitten. Why is my cat depressed? And what can I do to change that?