My fiance says: Even when you’re right, she will think she’s right. Always.
Honestly, have you considered your approach? I think you’ve gotten some really good responses here. Oftentimes you must change your OWN perceptions, ideas and actions in order for things to improve.
I guarantee that if you treat her opinions with respect and handle them with genuine interest she will no longer stonewall you. She will be open and willing to accept your perspectives if you treat hers with the same consideration.
If you feel like she is trying too hard to be “smarter” than you, it may be because she feels as though you don’t acknowledge her intelligence.