Am I going mad? Could this be related to diabetes or hypothyroidism?

No. Try to think more positive. Wake up and say this is a good day for doing stuff, like going out. Helps if it is sunny for me. A nice sunny day makes me feel alive and ready to go. That depends on your location. Instead of thinking you are depressed try to think that your are happy. Say I am happy today, and good thing will happen. Works for me. After all you are what you think and let no one tell you any different.