Living with Depression

I have had a depressive disorder since early adulthood…am now 62, my wife has stage 4 lung cancer and my 27 year old son has just ended a 9-year relationship with a girl I love as much as family. Lately I have had totally disabilitating fatigue, have cried daily off and on, and feel that once my wife passes, my purpose on the plant will be served and I will end my life. I feel as if my family and sons will understand as they aware of my deep depression thruout the years. I switched to lexapro about 3 months ago and it has just seemed to be non-effective. Was previously on Prozac and Wellbutrin with better results. Now I also drink a few glasses of wine every night…nothing seems to matter much…except the love I have for my dogs. I don’t know how much longer my wife has, and she just isolates in her room and doesn’t want to be around me. This is without a doubt my lowest point. If I had the energy to walk or do some exercise, I know that would help…but I am totally exhausted every single day. Can anyone share a similar situation…by the way, I was like this before my wife was diagnosed a year ago, so I don’t think it’s situational.
Thanks for reading and God Bless.