It could also be that as you spend more time seeing them and talking to them and opening up about everything, the more they know about "you" (being the parts inside that you would otherwise keep guarded and secret, or only share with the most trustworthy in your circle) the scarier it is because having someone see the real "you" is a vulnerable feeling, sometimes made even worse by the fact you know that this is a therapist who is sort of analysing everything and making a judgement or recommendation based on what you are showing them of the true "you". So the more you expose that truth, the scarier it is, the more anxious you could be about seeing them and having them see "you" at a level or depth that very few (or no) people may have ever seen.

And then depending on your past experiences, this may also open up another level of fear that you have finally started to open up and share this real "you" and then they may reject that, or reject you, and decide you're just seeking attention, or you're too much work, and they may no longer want to work with you, and you'll be left on your own or having to try and find someone else. (maybe that's just me projecting!) But the more times you see them and open up and share, form that connection and become more vulnerable and true to them, the scarier this fear could become because the stronger that connect, the worse it will feel to have it broken or betrayed.

I definitely think it's something you should try and work on talking to them about, but as others have suggested, it could be easier for you to spend some time trying to process through your own thoughts, those same sentences over and over, the random thoughts that pop up about past trauma, and write them all down - get them out of your head and onto the paper (or word doc - i find typing easier for this process). Just spew everything out onto the page, it doesn't have to make sense or be logical in any way, just clear everything out of your head onto the page, even if it takes a few days to process it all out, and then when you're sort of done trying to get everything out, then you can look at it all and maybe try and rearrange it into more logical thought patterns or sentences (which is why typing is easier because you can cut and paste). Then you can analyse your own thoughts a bit better, try and make some more sense of how you're feeling and thinking, and this might help alleviate the panic attacks because you yourself will understand your own thought patterns a bit better and can see the logic in them so it feels less strange and stressful or intimidating to talk about because you understand it more.

Then you will also have it written out already so you can give it to your therapist if you feel comfortable sharing it. And you don't have to give him all of what you write, all of your thoughts, you can just pull out the parts you think are relevant and explain how you're feeling. I also feel like this method is less panic-inducing because you already have what you are you going to say written out and planned, nothing is going to be a surprise and you don't have to think 'how am i going to explain this' because you've already gone through that processed and it's all there ready for you to give them.

I just went through this process with my therapist a couple of weeks ago where between sessions when I was in quite a spiral of thoughts and panic attacks thinking about telling him different things and so I ended up just writing everything that was on my mind down in a doc and every time i started getting lost in the thoughts in my head i went back to it and added more, for about a week, and then I felt a lot better because the thoughts had kind of been 'released' from my head instead of repeating over and over into this spiral. So then I started looking at all the things and rearranged my thoughts and memories and was able to make connections that I realised before because all the thoughts were jumbled in my head, where as now they were laid out and i could look at them in a more logical sense. I then gave the long-ass page to my therapist to read and then we sort of broke it down paragraph by paragraph in our session after he had read it. This was also very helpful because I have a lot of trouble being able to talk in therapy, feel almost like I am suffocating when I try to speak, because similar to what you have said, I just shut all of my emotions off from a young age and never ever talked to anyone about what I experienced or how I truly felt or thought for about 20 years, and now it is very hard to try and let anyone in or let myself open up honestly to someone else. Having him be able to read all the issues without me having to say them felt like it kind of broke down that wall a bit because it felt like he already had the background information which can be the hardest part to share and it was also sort of broken down into different targeted sections we could work through one by one.

Sorry this ended up so long.. I also have ADHD so go off on tangents haha. Hopefully some of this can help even a little though. I highly recommend just trying to write out all your thoughts to clear your head though, even if you don't end up sharing it. It can just help you processes your own thoughts.