As distressing as this is, I think it may be fairly common. There's nothing natural about the relationship we have with our T, and we have a lot riding on the relationship being productive and helpful. For me? It's mostly shame that makes it hard to get the words out of my mouth: even though I trust my T, saying the words out loud? To an actual person? And when I'm really unwell or there's a particularly significant issue that needs addressing, the pressure is even worse.
Personally, I think it's normal to get pretty uncomfortable talking about the kind of stuff that we have to talk about with our T's. They aren't the kind of conversations that people ordinarily have - in fact, often we're talking about stuff in therapy that we go to huge lengths to keep hidden from the rest of the world. And then we walk into our T's office and we're supposed to just let it all out?
When I know abead of an appointment that I'm going to have trouble getting the words out, or staying focused or present, I take notes. I write it down, and read it out, and that way I know that I've said what I needed to say. To date, I've never come across a T that has had a problem with that.
And often, I go one step further, and I take notes during my appointment of key things my T says - and sometimes my T will now suggest, "Do you need to write this down?" Because she knows that I'm super-stressed and that things like communication and memory aren't working as they should be!
Some people find, as an alternative, emailing their T before a session is a good way to let their T know "This is what we need to cover...". Find what works best for you and your T, because there's lots of perfectly legitimate ways to communicate with them other than saying stuff out loud, on the spot, in the pressure-cooker atmosphere that the appointment often is. The issue, to me, is not "How do I not be stressed about this?" because it's pretty stressful, and being stresed is ok, and normal. It's just about finding ways to communicate what you need to, that works for you.