I've always been a proponent of telling myself that, when I'm having a bad day, it'll get better and that tomorrow is a new day. And when I'm having a good day or good time in my life, I tell myself to enjoy it and to live in the moment.
However, years ago I was watching a movie in which one of the main characters had issues with clinical depression.
Her philosophy was that, when she was feeling good emotionally, she'd tell herself that she was doing fine...until it gets worse for her again.
I thought that was an odd way of looking at it. Like I said, if you're happy, enjoy it and worry about the bad times when they come.
Years later, a psychiatrist who I worked with told me that, when I was feeling good, don't tell myself that I feel fine until it gets worse again and, instead, just enjoy the happy moment and pleasant time in my life and worry about the bad times later.
I think her and I are both right about that.