Why can't I just be happy?

I feel not like myself lately. And I don't like it. And I don't know what's wrong with me. Before when I was not happy (for days not weeks) I would at least felt content. Now I don't even feel content. I am easily irritated, bitter and annoyed, by stupid things. I hate feeling this way because I know that my spiritual defenses are weakened, and the devil works when people are weak. I just pray that God will help me through this and keep me strong. Please pray for me as well...