unsupportive girlfriend

I find it interesting and annoying how my
girlfriend in most cases doesn't take my side.

Yesterday, for example, I told her that my new dentist doesn't have
a clue when it comes to periodontitis (I was diagnosed with it
a few years ago by a German dentist) and now this new dentist in
Croatia tried to tell me and convince me that I don't have
it even though it has gotten worse and it's visisble (receding gingiva/gums). He just looked
at my teeth for less than half a minute.

My girlfriend told me when I recounted her what I think of
my new dentist that maybe he is right and that maybe the
periodontitis has improved. I told her that this was not the case.
On the contrary, whenever I eat something very sweet or very cold my
teeth hurt horribly.

Why can't my girlfriend just accept that I'm not wrong and that the
dentist isn't right. She very often assumes that
I'm too critical or that I'm mistaken. This makes
me feel misunderstood and I certainly do
not feel as if she morally supports me or
as if she is fair towards me because I can
and do praise people when I deem that they have done a good job. Moreover, she told me not to discredit him even
though she is also his patient and wants to choose a new dentist as soon
as possible. She is often dissatisfied with his work and
his treatment methods and she complains how he wants to remove all
of her teeth while other dentist try to save as many teeth as possible.