Throughout the day, take “minibreaks.” Sit down and get comfortable, slowly take a deep breath in, hold it, and then exhale very slowly. At the same time, let your shoulder muscles droop, smile, and say something positive like, “I am r-e-l-a-x-e-d.”
When you are feeling badly, you are also often feeling out of control. Your internal state is often a reflection of your external state; when your environment is messy, you feel messy inside. Time spent organising your space is time spent building up your sense of control and accomplishment.