Freezing when you sit down to work by yourself is the same thing as freezing in front of an audience. In both cases you're crippled by the fear of what people think of you. The only difference is that here, the audience is in your head, a hypothetical group of people that will judge your work in the future. You can almost feel their eyes on you while you try to work. How can an imaginary audience have such a paralyzing effect?
The answer is that deep down you feel you have to be perfect to win validation. That's impossible. In fact, there's a strange truth about human creativity: The most creative part of you is also the most imperfect. This part of you is called "the shadow."
When you try to be perfect in order to win the validation of the audience, all you do is freeze your Shadow out of the creative process. You've silenced the most creative part of your self.
The solution is to invite the Shadow back into the process—to identify and accept it. This requires you to accept the worst, in whatever form that comes: write the worst sentence, paint the worst portrait, play as off-key as you can. Once you do this, the Shadow feels accepted and its creativity will take over.