I'm going to break up with my depressed girlfriend.
Why do you want to break up with her?
I don't like that she's never happy.
Are you sure you don't want to confront her with her depression and try to help her through it? Would you want someone to quit you because you were depressed? Maybe if you love her you should support her.
I think it's better she dated someone else that's more happy.
You need to do what you can to get her through this. Maybe you can get her some help.