It's very important to make her see it in the sense of, you think about her a lot and you want more of her. Don't let her think that you are just after pure sex, because she won't feel special then, she'll approach sex as "something that has to be done just because we live together" and this can make her not want it as much anymore.
You complain about oral, but do you give her oral more than 3 times a month? Do you do what she likes or are you mainly frustrated because she doesn't do what YOU want? (I'm not saying you don't do anything for her, I'm just trying to figure out both sides of the coin). Trust me, I understand your frustration and going to the doctor shows that you do worry and want to fix this, many men hesitate to do that (I know mine does) and I agree with the doctor, it's clearly a mental issue.
If she hasn't been sexually abused in the past and her attitude just changed (and she is not cheating on you) then it must have to do with sex becoming something habitual or her not feeling you want her in the same way as you did before. It's definitely mental for her too and you have to find out what is wrong with calm discussion where it's going to be all just about HER. Not because your feelings count any less, but because she obviously has a serious issue going on and the only way for her to tell you what it is is by not judging her and listening to her as much as possible, without saying a word about your feelings (at least during one discussion). It's important to her to know that you love her anyway, whether the sex would be more or less. Once you find out what the problem is you can decide what you want to do, either stay and solve the problem together or leave because of bad sex.
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