If you tell your self you are a failure, or you WILL be a failure, then you are more likely to succeed at that.
Why not turn it around and say "I CAN do this, AND I WILL succeed!" Then you are putting yourself on the path for success.
Where it comes to weight loss, just start with baby steps and you WILL succeed. Don't try to make all the changes at once, otherwise you won't succeed.
As Meghan has suggested, talking with a Therapist or Clergy will help you. You could talk with your Dr about what you are wanting to do and the struggles you are having, and ask for a referral to a Therapist who specializes in these issues. Also, Registered Dietitian's will be able to help you as well and your Dr will be able to refer you to one if need be.
I am sorry you feel like you are bad at life. I am sure you aren't. We all have days like that. Really! :-) If you feel this way often, speaking with a therapist or even a clergy member may be beneficial.