She used to let me do that when we started dating, she would also go down on me a few times back then.

Then suddenly she stopped letting me doing it, said she has some “psychological difficulties” letting me do that, that it’s “bad for me”.

I really tried persuading her that I enjoy it, and it’s not “bad” for me in any way.
I tried to find out if I was doing anything wrong, perhaps something she doesn’t enjoy? but she says that it’s not that. just that it’s “bad for you to do it”.
I make sure she always understands that my only goal in sex is to make it enjoyable for both of us, and i don’t want to pressure her into doing things she doesn’t like, I myself never asked her to go down on me (and I probably never will).

But what bothers me the most, is that we did use to do those things, and she claimed at the time that she did enjoy them, then all of the sudden she doesn’t like it anymore. I feel like the “psychological” reason is perhaps partially true, and when I try to talk to her about it she says things like “just deal with it”.

What is my best course of action? I want her to feel as comfortable as possible with me, and it will make me very sad to find out that I did something wrong and she wouldn’t tell me what it is.