That small voice in your mind is your "inner-critic" he/she told you that what you're writing is worthless and won't get anywhere. That voice is trying to convince you that you can't write and what you just wrote needs to be revised RIGHT NOW. You feel upset, confused, disappointed, and frustrated. You probably thought that what you wrote was perfect, well at least semi-perfect, but then you were torn down with those critical thoughts. But whose thoughts are they and whose words are they?
None others but yours.
It is your very own mind that is telling you these things and of course you believe it. Because obviously if you don't listen to yourself you just might make a fool out of yourself, right? It's always better to listen to yourself the first time through than make a fool out of what you just said--maybe? Well don't feel like you're the ONLY one who has done this. Believe me, when it comes to anything that we do, say, or think, we are our very own best critics. We are either too self-centered or insecure about what was just done. Okay, so how does this relate to writing? Easy. We write and our words go public if we share it. But before it even gets there we think, "Wait a second, does that make sense? No. I don't like this either.." So we can be blindfolded and our fingers know exactly where the delete key is.
It's time that you STOP listening to your inner critic. When you write you need to turn off that small voice in your mind and tell it to come back when you're done. There is a time to rewrite what you just wrote but there is also time to just write and come back later to your work. When you write you should just write. Don't look back at what you just said and don't think how you can fix it. Save that for later. Right now you need to just write and worry later. Your inner critic is strong. It wants you to believe that you can't make it out there in the writing world. If you listen to that stubborn voice you will soon believe it. You have to learn to fight against it and go with what your gut is telling you--YOU CAN WRITE.
As a writer, or a person in general, you will always feel under pressure and criticism. Criticism comes with living. It comes as a second nature thing for writers. If you cannot block it then I fear that you'll never get anything written down. There is a time to listen to that voice but also a time to say, "Not right now! I have to write." You will not know if your writing is worth it until it's all written out and if you constantly erase how will you ever know? Ignore! And write!
And it might just help to know that mistakes will come. You're a human. Did that surprise you? You're not perfect, but you can still refuse to listen to anyone else telling you that what you're doing is senseless--even yourself.