From today start noticing the small things you achieve, there is no need for a major achievement to take note and celebrate.
A simple walk, helping someone, reading a book are all achievments you can be proud of. When you make a mistake instead of listening to your inner voice telling you that you are not good enough stop and ask yourself what would you tell a friend you made the same mistake.
Would you tell him he is useless or tell him that we are human and everyone makes mistakes?
Learn to be kind to your self and by time you will start feeling better.