One link between depression and creativity is rumination. Mulling things
over, following lots of mental paths, etc., can result in creative ideas
or depression depending on which way your brain turns. One part of being
an optimist is not dwelling on bad things very long, keeping busy. One
part of being creative is thinking about something a lot.
One thing you could try is working harder on other parts of optimism
(bad things are temporary, limited, and not your fault; good things are
lasting, significant, and your own doing) so that you can afford to spend
some time in unstructured thinking. Other things you might try are picking
particular things to be creative about, reading a few of the excellent
books giving methods for being creative, and doing enough new things
in rapid succession that you mix up your conception of the world a bit.