I feel depressed and like nothing matters... what should I do?

I feel emotionally drained. I felt fine until this sudden smack of depression hit me. I was listening to music and some songs made me thing of my friend that passed away. I was on my computer looking at pictures of He and I and I feel so broken now. People say my life is looking up because everything is going fine, but I don’t feel that way. I feel like I lost a huge chunk of my future. I had a fantasy with my friend that we were married and lived happily with our family together. I just want to die half of the time. I feel like my life is just dragging along uselessly. I keep focusing on the absence of my friend when I should be trying to focus on the good. I sound like I’m obsessing over my friend… which I am, but I loved and still love my friend a lot. What do I do? I can act happy, but I want to die!