Hi Gracez,
what I do is prepare.
when I am having a good day, I write it down, describe how it feels, acknowledge the fact that good days DO happen, and that they are nice.

Then, when I descend back into that pit of doom again, I reread what I had written. It may not necessarily 'cheer me up', but it helps to remind me that good days happen and that the next one could be right around the corner.