Everyone has their strong points and their weak points. 

And they’re likely different to yours.

Don’t lose faith in someone simply because they’re not good at stuff you’re good at.

The mistake is yours. You’re asking her to play to her weaknesses. 

Rather give the support needed for her weak area, so she can put all her energy into her strong area.

For example, you have a sales lady who is excellent at getting meetings, and closing deals. But she’s terrible at admin, so she’s regularly late for meetings, and doesn’t have the paperwork ready to close the deal.

Instead of getting upset, give her an admin assistant. 

Someone to make sure she’s always on time, and has all the documents at hand to close the deal.

That frees her up to be a super-star, and takes the stress away from you.

Play your people to their strengths.