I can be one of the most positive and forward-thinking people out there. That said, I'm here to tell you that being "gloomy" is okay, too. Sometimes people need to hear the truth, whether they like it or not. Don't be afraid to tell it like it is. People will trust you more if you are open and honest.
If things are flowery at the moment, then fine, be all rainbow-sprinkled-unicorns with purple magic fairy dust flying through the air. But if that's not the way it is, you don't need to pretend it is to produce writing. In fact, the best writing will speak the truth, no matter what that may be.
When you always speak the truth, whether it is fluffy or gloomy, your readers will learn that they can trust you and come back for more. Beyond that, you can always be confident and satisfied in what you are doing. Be gloomy if it's necessary for the situation. When what you speak is the truth, you'll feel much better for doing what you do.