You have tried to motivate this employee positively, and the results have been negative. Now it is time to do more than just talk.
You should meet with her to review the specifics of her most recent questionable behavior, and then jointly establish some specific steps to improve her performance, along with some measurable goals to be met within a given time period. Let her know that you will be there to coach her along the way. She also needs to understand the consequences if she fails to show improvement.
Putting all of this in writing is not meant to be threatening. Rather, this approach has advantages for her and for the company. It gives her a clear idea of the total game plan, and if you have her sign it, you may actually increase her commitment to making a real improvement. And if you do need to take disciplinary action, you will be in a better position to do so if you have documented her performance and involved her in the plan to correct it.
You seem to be concerned about having this employee like you. That's more likely to happen if she can sense growth and accomplishment on the job.
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