Write down when you have a good day and how you feel. When you are feeling depressed, read it. Then you will know that however bad you are feeling when you read your journal, you will soon get a good day again - as sure as night follows day. Rooting for you Ed. What Hi Ed.
Bad days suck.
I think Angelica's suggestion about journaling is a good one. It will allow you to go back and see if there's a pattern to your mood changes. I know these things effect everyone, but it shows up WAY more in him than it does me, I have noticed patterns. You might want to keep track of these things to see if there is a pattern for you too. Some people seem to be more susceptible to these factors than others.