"I'm not an artist - I'm a phony
I have nothing worth saying
I'm not sure what I am doing
Other people are better than I am
I'm only a [student/physicist/mother/whatever]
I've never had a real exhibit
No one likes my work
No one understands my work
I'm no good"

"The first few brushstrokes to the blank canvas satisfy the requirements of many possible paintings, while the last few fit only that painting - they could go nowhere else. The develpment of an imagined piece into an actual piece is a progression of decreasing possiblities, as each step in execution reduces future options by converting one - and only one - possibility into reality. ........The irony here is that the piece you make is always one step removed from what you imagined, or what else you can imagine....."

From Art and Fear

This is so true. I am constantly fighting the negative thoughts and long to be one of those artists who have faith in themselves and their ability - and yes, there are some!

As to the second quote - I find this all the time. It used to frustrate me extremely that what I had in my head never appeared on paper/canvas. SO now I try very hard not to even try to imagine a finished result but rather to only have a concept - that way allowing it to appear organically - and then try to fool myself that what appears is what I really wanted!