So why do most people not take a chance to do comedy? The vast majority of us say something like "I could NEVER do that!"
Why is that? It is fear. It's the fear of looking ridiculous, bombing - etc.
Fear is all in your mind. You can react to it one of two ways... you can either let it paralyze you, or you can take action anyway.
Instead of visualizing yourself doing bad, try visualizing yourself having the audience laughing hysterically at every joke.
You can apply this same strategy to other areas of your life as well. For instance, if you are a guy afraid of talking to women, visualize yourself being an alpha male and having lots of women completing over you. When you actually do the approach, women will sense your positive energy and react positively to you as well.
Fear is only bad if you let it hold you back. If you take action despite fear, you will push your limits, and become a better person.