As a psychotherapist and parent, I have found it extraordinarily helpful to craft catchy little sayings to convey concepts I hope children will internalize. You see, when I am working with adult clients, I notice we spend a whole lotta time undoing beliefs (and related feelings) that people formed as children. Why not choose beliefs that are helpful, loving, and kind to "install" in our children's minds? Sounds kinda like brainwashing, but if we are doing it anyway, why not be conscious about it? And believe me, children are internalizing the messages they perceive you to be communicating.
One of these sayings that has been so helpful over the years is, "If you think you can, you can! If you think you can't, you can't!" I encourage children to experiment with it. Try doing that task you are struggling with while saying, "I can do it, I can do it, I can!" Try doing it while saying you "can't". What happens? To add to the lesson, depending on the child's age, I will say, "Your beliefs are very powerful."
Here's an example: Five year old Tiara is attempting to tie her shoe. She learned how to tie them last week. Her pink shoelaces are floppy and her bows are so big, she doesn't have enough of the long part to finish the job. Tiara says with a frown, "I can't do it!" I squat next to her, and start the conversation by mirroring to help her feel heard and seen, and to de-escalate her. "You are having a really hard time tying your shoe. It seems like you can't do it!" She nods, and exhales. "You might be surprised, but what you say to yourself is very powerful. If you say you can't, then usually you can't. But if you say you can, then usually you can! Try it. Just keep trying to tie your shoe, while saying, "I can do it, I can do it." If you think you can, you can. If you think you can't, you can't!
Tiara is game and she quietly repeats the mantra while working with the laces. Eventually she does it (I sit with her to support her steadfast trying), and looks up at me, beaming, "I did it!!" She's hooked. If you think you can, you can!