It's a complete falsehood that you have to be positive all the time. It's only natural to imagine the worst at times, not all the time but sometimes.

So never think, even just for a moment, that you have to be upbeat and positive when life isn't going to your way. You are far better off acknowledging things aren't going so well, talk to someone else and reason what is happening out, than putting on a false face just for show.