What Causes Severe Mood Swings And Lack Of Motivation To Do Work?

I feel really low all the time. Sometimes I have short periods of being super happy but I don't know whether I actually am or am trying to convince myself that I am. I have such a lack of motivation for anything, even things that I enjoy, and I think about harming myself and what it'd be like if I was dead. I cry for no reason and have no reason to feel like this. Last year I had huge bouts of severe low mood which resulted in self harm. I don't want to get to the way I was again and I think I may be depressed but in not sure. Please can somebody help?