"I've found that it's of some help to think of one's moods and feelings about the world as being similar to weather.

Here are some obvious things about the weather: It’s real. You can't change it by wishing it away. If it's dark and rainy it really is dark and rainy and you can't alter it. It might be dark and rainy for two weeks in a row.

But. It will be sunny one day.

In the same way that one has to accept the weather, so one has to accept how one feels about life sometimes. "Today's a crap day," is a perfectly realistic approach. It's all about finding a kind of mental umbrella. "Hey-ho, it's raining inside: it isn’t my fault and there's nothing I can do about it, but sit it out. But the sun may well come out tomorrow and when it does, I shall take full advantage."

Source: Stephen Fry, It Will Be Sunny One Day (edited lightly for clarity)

Check out his documentary on bipolar disorder if you haven't.