I have noted that how I feel at any given moment has damned little to do with how things are really going.
Of course truly bad things happening, or good, will change my mood.
But when there is no good new or bad, my mood still changes dramtically from day to day.
I am of the opinion that people have a happiness level generally, toward which they naturally gravitate regardless of what is going on in their lives.
Abraham Lincoln once said, "People are about as happy as they make up their minds to be."
He was right. If you're having a bad day, put on some happy music, close your drapes, and dance to it. You'd be surprised how quickly your mood will change.
That is wonderful that that works for you, good job in finding what can break the cycle for you.
I fear this would not work for all people though.
Many have very different sources of their depresssion and or sadness. Every person is different and we all grieve differently. Depression is a tempory condition but it can last for years. The best thing is to seek help if a solution such as you recomend does not work. Never ,never stop believing its temporary and seek a way to the next day.