Here is something to consider when you feel bad and have zero motivation. Positive thinking is more than just feeling good or a hypothetical nirvana, it is physically good for you. There is now scientific data on many levels to back all this up. Lets start with just the negative emotions.
There are many studies that have shown just how much damage negative emotions do to us physically. An article called, "Cultivating Positive Emotions to Optimize Health and Well-Being" by Barbara L. Fredrickson displays this. Fear and anxiety can lead to psychological disorders. Sadness and grief can lead to imunosupression. Unmanaged anger can lead to damaging effects on the heart.
Now on to the positve emotions. I stumbled upon an article written by Siri Agrell that highlighted some very interesting things. "Psychologist Robert Emmons studied organ-transplant recipients and found that the more gratitude they felt, the faster they recovered." "A 2001 study of trauma survivors by psychologist Russell Kolts found that gratitude was associated with lower symptoms of post-traumatic stress disorder." "A Wellesley College study that has tracked 200 people since the 1920s, interviewing them for five hours every decade, found that people who were charitable in high school had better physical and mental health in late adulthood."
How can you break the cycle of being stuck in negative emotions? Here is a quick fix: music. Put something on that makes you really happy and dance. This will shift your focus and your brain will start to create endorphins and you'll be getting your heart rate up. All that makes you feel good. I had a great-grandma who lived to be over 100-years old and she was always a positive person. Everything was always, "lovely dear." Apparently she knew what she was talking about.