Well... ultimately I think the only way to do this is to do it. As someone wrote in a post I read on another website: "Rip the band aide off & let the healing begin." But I do understand your concern.

I'm an old man now. But one of the issues (among others) I've struggled with my entire life was gender dysphoria. I never breathed a word of this to anyone until, around 20 years ago or so, I decided to talk about it with a therapist. (My situation was a bit different from yours, I presume, since I was seeing this therapist for the first time; & it sounds like you've possibly been seeing yours for a while.)

Anyway I was just too mortified to bring the subject of gender dysphoria up in person. (This was a lot of years prior to Caitlyn Jenner.) So, in advance of my appointment, I wrote him a letter. On the day of my first appointment the therapist asked, of course, why I had come to see him. I mentioned the letter whereupon he reached into his file drawer & pulled out my letter still in the sealed envelop. He then opened it & read it as I sat there squirming & feeling thoroughly embarrassed in a dozen different ways.

I'll spare you the details with regard to where my experience went from there. (It wasn't far.) But I recount this experience as a way of telling you I know, or at least I think I know, something of how you're feeling. Writing that letter did not turn out to be a highly successful approach in my case. But it might possibly be a way to break the ice, as the saying goes, for you. Another option, if you write a blog or a journal, might be to give a relevant part of that to your T to read. However, in the end, you simply have to pluck up the courage to throw open the doors & windows & let the sunshine in.

One thing I think is helpful is to come to the realization that, whatever type of fetish it is you have, there are no doubt lots of other people who have it as well. It's a great relief when you realize you're far from alone. So if you haven't looked around on-line at fetish-related forums, that might be something that would be of help to you. I know, in my own case, I lived for decades believing that I must be the only person in the history of the world that had been saddled with the burdens I carried around. It's only been within the past... oh... 12 to 15 years or so I've gradually come to realize that none of what I've dealt with is in the least exclusive to me. (As Plato, I think it was, supposedly said: humankind has not had a new idea since we first walked out of the cave.)

One thing I would caution though is to be a bit careful about this. If you know your T well & have confidence in her / him, then all should be well. However therapists can be as closed-minded & prejudiced, or just plain ignorant, as anyone else. It is within the realm of possibility that the response you'll get from your T may not be what you expected... either for good or for ill.

I don't know if this will be of any help to you, but here are links to 4 articles, from PC's archives, that I thought might be interest:

6 Ways to Open Up and Talk in Therapy

5 Ways to Prepare for a Difficult Conversation

Tips for Talking About Tough Topics

Effective Conversations About Difficult Issues | The Emotionally Sensitive Person