There is no point in keeping all the negative memories or holding grudges because in the general picture of your current state it has no sense. Maybe, it was something really worrisome and horrible yesterday but it does not mean you have to keep going with it today. Leaving all of the past unpleasant experiences behind and moving forward with enthusiasm and hope are the key for a happy being. If this one habit you can master, you will see your happiness-meter go up.