All I can say is that I was depressed for decades. Finally I \found a dr. who worked with me long enough to find the right medications for me. Once the depression and anxiety were under control I was better able to think and evaluate my life. As I gradually got involved with children I found that even though I don't do much, even that small amount was enough to make a child's life better. That gave me courage to do a little more. So I feel useful and I think feeling useful gives us motivation to live. Gradually you don't think about yourself all the time and start thinking about how you can help. Then you get a little freedom from your sadness.
At least that is my experience. Also I take my meds!!!!!!