It's only a piece of paper

Tuesday evening sees me at my watercolour class. I am increasing in confidence and enjoyment and am beginning to think more about the style in which I paint rather than just hoping that my picture will be half-way decent.
We have just done a two-week project and were encouraged to sketch out our work first. Many of us joked that our drawings were better than we thought our paintings would turn out. One fellow student, an accomplished artist, did a beautiful charcoal sketch but was left with the feeling that, as she painted, she was not capturing the spirit of her drawing. Our tutor suggested a new technique - painting a colourful wash and then lifting out the paint with a rag to leave highlighted areas - which required both bravery and quick work. Not an easy combination. It was scary. "It's only a piece of paper," encouraged our teacher.
As the evening continued, I pondered this. Yes, it is only a piece of paper, it is only one evening and a few marks of paint. Why does it feel so much more? Why do I experience, as my fellow student clearly was, feelings of nervousness, even fear, when I embark on a painting? More than a piece of paper, it is an investment of my heart; it is venturing out into something new, where failure is a real and present danger and open for all to see. It is taking an infant idea, vulnerable and fragile, and trying to work it out in concrete form with limited skill, exposed to potentially critical eyes.
I have had the joy of watching three human beings learn to walk. It was not something I tried to teach them or even encouraged them to do: it is inherent in our nature that we will stand, and walk and run and even, one day, hop. All three of my children have consciously and without outside suggestion taught themselves to hop, beginning with jumping and moving on to holding on to door frames. They have all proudly shown me their attempts with no fear of my criticism and I have responded with delight and enthusiasm and never once pointed out that hopping is supposed to be on one leg. And they all hop pretty good now!
When did I become so afraid of failure that putting paint on paper is a nerve-wracking experience?