Some people have to be right, others have to win, some want to advise, others feel obliged, and put upon. You say you feel like you are losing yourself. That could end up with you feeling like a victim which is not an acceptable outcome for anyone involved.
There are several ways to go here. However, once you overplay your hand your complaint loses some of its validity and options. One path would be not taking her need to be right or have the last word so seriously. Remember when you thought it was cute? Have you tried saying, of course, you are right dear; you are always right and let it go at that, or are you tired of saying it? Be clear with her about how you feel and communicate it.
Whenever one uses expressions like always or never, it implies a bias that may or may not be completely true. Not even the customer is always right. It may be better to tell your girlfriend you feel like you are losing your ability to assert yourself with her and it is important to your partnership that she listens to and acknowledge your needs. See what she has to say about that and discuss your point of view.
Sometimes we learn these behaviors from our parents. I acquiesce easily preferring harmony over conflict, and teamwork over the competition. I can see from here that proving her wrong is not to your advantage. You have no intention of leaving her. You can be right or happy. Since she is right, why don’t you concentrate on being happy and see where that path leads.
Yes, my dear, there are worse things than being occasionally wrong, and one is losing the love of your life over semantics. In other words, who cares, kiss her and go dancing. Make sure you lead. And remember attitude is everything, but you can only change your own attitude not anyone else’s.
Peace & Love,
PS: It’s a big burden to always be right, make sure your girlfriend knows you don’t expect it.
PSS: In the case of overbearing tyrants—run or flee immediately!