Niece has extreme perfectionist attitude lately

We had my 7 year old niece for a sleepover last night with the idea that her and I would do some painting together. My mom (her grandmother) has lately described situations where my niece does not even want to try to do something if she isn't good at it but I didn't realize how bad it was until this morning. Together we chose a picture of a unicorn on the Internet that we would both work from and as I got started on my sketch she did about two lines and then really quickly erased them and stopped stating "it's too hard". She refused to even try after that insisting that she "already tried"and it's too hard" . This turned into a very big deal where she cried and threw down the art tools and got really upset. I would like to be able to help her through this and show her that we try things and then practice to get better at them... Does anyone have any advice on how to correct this thought process for her?