2 things I say to myself when I'm feeling down in the dumps:

If life was easy, then everyone would want to live it.

If it wasn't for the shitty times in life, you wouldn't know to appreciate the good times.

Bonus, for more specific incidents of anger or sadness:

Thank god she left.
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Thinking of cracked-out and/or tweaking whores getting their throats and asses brutalized for the next hit makes me hard. --Rear Admiral