|Although you do not want to lose this employee, her actions are causing you to lose respect, credibility, and time. She probably realizes that her skills are quite valuable, and this has led her to think that she has a wildcard that allows her to give you orders, insult you, and steal your credit.|
These behaviors do not appear overnight, and since you have done nothing to stop the erosion of your power and position, she has continued to gnaw away. She is going to continue to do this until you act more assertively.
You may be performing well as a manager in other areas, but when it comes to managing her, you have completely abdicated. Granted that you value her skills, you should remember that she can leave at any time. Perhaps you should be building some bench strength under her in order to prepare for that day.
In the meantime, meet with her at the next sighting of one of her inappropriate behaviors, let her know exactly what you think, and tell her the consequences if it continues. She needs to realize that she is not indispensable, and unless she starts focusing on doing her job rather than undoing yours, she may well be focusing on another job altogether.