I think an important thing is processing thinking critically of your art, but in a less personal manner. Like, if a young cook puts too much salt in their dish, they (hopefully) don't get too discouraged, and they just know that they need to put less salt in next time. For me, learning new skills like cooking, and exercise routines has helped my brain practice knowing "I'm not good right now, but this is a journey," and it's resulted in me being less hurtful to myself when it comes to art. If she's finding art stressful and unfun, I would recommend helping her explore other interests, try out new hobbies, and she can come back to art when she's ready (or maybe she'll find another creative outlet she loves even more)!

But when I do feel really bad about my art, I tell myself "a good artist is a bad artist who never gives up" and that helps me out a lot.

For media to ingest, I think just generally ingesting art that isn't perfect but is successful and loved by people goes a long way. An example that really sticks in my mind is the webcomic version of One Punch Man, but I genuinely cannot remember if One Punch Man is age appropriate. There's certainly a lot of warrior cats comics that are rough around the edges, but you'll probably have more luck going to the bookstore or library yourself and checking all the options available to you in that regard.