[infogulch]

The problem is that long-term sadness and depression feel very similar and they can be difficult to distinguish. Sometimes you can feel sad about some situation in your life and not even realize it. Or feel sad about something but you're suppressing it because of social pressure. These can look like depression.

Take a really hard look to find things that are making you sad. Especially things you're telling yourself that you're not sad about. If you find it, fix it like GP says. If you don't find it, pursue depression treatments such as the post.