Thank you for wishing me well!
I did scramble the courage together and went. It was kind of heart-warming because even though I thought I hadn't managed to talk to the part who I wanted to go, he was there in the morning anyway. Then while travelling anxiety got too much, and I fronted again.
I did lots of breathing exercizes, but I couldn't help feeling nervous and dissociating anyway. It was very scary.
Then at the start of the assessment the lady asked to record (video) it, and that made us feel a little more anxious still. If we didn't agree, she said we would maybe need to come back and re-do the assessment. So we said yes.
Thankfully, she was respectful and I could tell she was trying to make me feel comfortable. That was nice. There were lots of questions. Of course I don't know what she thinks... but at the end she said that she thought that me and T are doing great work, so that does feel comforting anyway. I don't really mind the diagnosis. I just wanted to join their group... But the group is pretty much off the table, as it is already full anyway. So now I'm mainly focused on diagnostics and I am curious what they will say.
She said she will write out the interview and my answers and then look back at the video if she has doubts/can't remember properly. And then if she doesn't know how to interpret the answers she will ask a colleague to look at pieces of the tape, too. The process of diagnostics and discussing with colleagues will take some weeks.
She was nice. I could tell someone inside wanted to ask her if she couldn't just take me home.

Thanks for your support, guys. I am super tired, but relieved that it is over.