"No matter how you feel today, things will be different tomorrow." Dr. Nicely (my former therapist) told me that. It was powerful. For the days when everything seemed to be gray and nothing was going well, I had faith that tomorrow would be better. And on the days where I was as high as a kite, it kept me grounded and nimble and never too blinded by my euphoria.
I pretty much say it to myself every day and it always feels positive to me.