Dear Ms Spock, I have some of these same problems. I too am hard on myself and often think of myself as a total failure. None the less, when I come to my senses, I realize that I am only human and that I can only do so much. SO I try to look for little things that I did right. I take it to the bank when someone tells me that I helped them. I buy myself flowers. (This is important, because when no one else ever buys you flowers, you need to buy them for yourself and write yourself a little card that thanks you for being good to yourself or doing something right or something). I have done this before, and it makes a difference.
Call a friend, someone who is close and honest and caring and ask them what you have done right in their life for them. Ask them if you have helped them at all and how.
Write a list of things you can do to help others and to help yourself too. Then check the things off your list once you have done them. You can even ask folks what they need from you and then do it. Don't forget about all the random acts of kindness you have done!
Personally, I think you have done a lot for the folks on this Forum by leading the challenges you have lead. Also, the ones you have participated in, just your presence in each of those groups was a help to others including me.
Also, when I run into you in chat, you cheer me up with a hug and whatever you say. So thanks.