here's an exercise that was suggested for me to do about the same feelings that i had about myself. i was told to draw a line down the middle of a page. one side was to be titled "my assets", the other "my liabilities". fair warning it's not easy to see any good things about yourself when you feel down but i worked on this paper for days and was amazed at the result. like, are you a loyal friend to others? that's an asset and so on. do you expect too much of yourself, but don't expect others to be as "perfect"? that's a liability. i had to be brutally honest with myself to finally put down some assets. as the time went on, my assets outweighed my liabilities!
i hope this suggestion will help you, too. sometimes we are so in the pit of darkness that we cannot see the positive things about ourselves. it can cloud our perception of what is really the truth.