The self doubt you face is similar to my own. It is the most difficult piece of depression I suffer from. I can't be good enough for anything and I'm no good for anyone, according to my train of thought. It's rough.
I've been a volunteer support aid before and did it for a while. At first, I did it for people who had been diagnosed with cancer. I've had my own fight with the illness and figured I could use my own experience and understanding to help others, doing that didn't really ever get to me. It was when I became a support aid for sufferers of depression, anxiety and PTSD that I felt myself sinking. I suffer from all three myself and figured that I could do the same as I did when I had cancer. Thing is, it wasn't the same. It took a toll on me that I didn't see coming. I gave up fighting to look happy. I went for looking neutral and stable when dealing with professional situations. Personal relationships were where I went for "happy". Only ever told a few people where my head really was. More than all of it, the difficulty was embedded in my unrelenting mindset that I was no good for/to anyone. I still suffer with it. At this moment in my life, I have been hiding from everyone the likelihood that my cancer has relapsed and just how sick I feel all of the time. I don't want to scare anyone unless I know for sure what's going on because it is my job to take all of the bullets for everyone else. They don't deserve to suffer with me. No one does. Yet, I feel like I deserve every excruciating minute.
Sorry, I've kind of run away with it. I wish I had advice to help you through this other that the usual "get help" banter but I don't. Good luck sir.