[sillypudding]

You may have explored this already, but just in case you haven't: get checked for chronic/lifestyle diseases (diabetes, fibromyalgia, IBD/Crohn's, etc). Many of these sap out your energy chronically, making you feel gloomy/depressed/irritated in otherwise "normal" life-situations. Even if you can't cure these, being aware of them lets you set realistic expectations about your body's limits (like, for example, you might NEED 10 hours of sleep at all costs, limiting the time available for other activities).

Also don't forget that physical and mental health are closely interlinked such that if you aren't doing well physically, then your mental health will suffer and vice-versa.