Here's a challenge for you today (or try it for a week for better results. When someone asks you how your day (or night was, only answer with the positives. No complaining about bosess, co-workers, traffic, the line at the coffee shop, the housework you had to do or the fight you had with a friend.
Try answering with only the good parts of your day: seeing a friend at the store you haven't talked to in ages, the nice walk with your spouse after dinner, the invitation to the much anticipated wedding you received.