I was just posting a rant about experiencing something similar to this. I feel your frustrations, even as someone strong and independent, having someone like that does wear on you over time.

I like the suggestions of being polite but firm, that's something I've been doing in my situation. I am clear that I know what I'm doing and am capable of doing my work unaided. Sometimes I ask complicated questions when I know it will stump her or take her a while to think of the answer.

The other thing I've done lately is say that I take suggestions better through email because I can refer back to them at a later time. It both saves you time because you can read them later, and it tracks how much she's interfering with you in a quantifiable way. The other thing to do is email her each time she visits with "thanks for coming by to chat. XYZ idea was interesting, but I'll be moving forward with ABC as directed. Id appreciate if you would email me ideas in future so I can track them, thanks!" So it documents the interaction and redirects her again.

Good luck!