Remind them that before they can get "good" at anything, they have to practice and work hard. It is okay to not be an "expert" at something right away.
Sometimes, kids (and adults as well,) get a little too comfortable with what they know and are good at, and when a new challenge comes along, they can get easily discouraged. It is important to let them know they are doing a good job by trying and not giving up and that with time and lots of practice, they will eventually conquer their new goal - no matter what it is!
It is our job as parents to encourage them and remind them that before anyone becomes an "expert" at anything, they were once a beginner! Teach them that today and every day, is a great day to learn something new!
"The Expert in Anything was once a Beginner."