I'm a bit drunk so forgive me if I don't make sense, because it makes sense to me xD

Had a fun day out with my friends today on Brighton Beach, but the whole time I still felt depressed, still feel depressed, and I've hit the bottle again when I came home.

Why do I feel this way? I should be grateful I had a good day, but I don't feel like that.
I mean yes I am grateful that my friends invited me out for the day, but I'm being ungrateful because I should be happy.