I’m writing fiction. I thought her weakness would be she feels superior to other people. Since she is usually right, she thinks she is always right. That is still not a weakness to me.
It could be a physical weakness of which she is embarrassed due to ridicule in elementary school. Maybe she was born with some deformity so that she needs a cane or walker or braces to get around.
Another point. People who are usually right don’t necessarily feel superior, nor do they necessarily realize that they are usually right. Why? Because people around them resent them for being so “perfect” or “right,” and therefore make fun of them and criticize them at every turn, causing them to feel inferior and “wrong” or stupid.
But you say your character is very confident and mentally strong so I guess her life experience has been positive. In that case, a physical weakness might work. Or a speech impediment. Or a creepy pet (or relative) in the house she wants none of her friends to see but whom she loves dearly.
Bottom line: People who appear strong and confident on the outside normally have a very soft and vulnerable private side.