I don't know if it helped/might help at all but I do hope the blog I sent you by my brother in law can be some comfort to you over this difficult period.
From what yoy describe, your situation has so many parallels with his: he too faced a diagnosis of progressive severe disability, he was also around 30, he was also single and he also suffered with depression. As you know, he received the worst possible news, and he had to grieve the loss of how he thought his life would be. He then realised he could still find space for himself, and parkinsons disease didn't have to define him. He still had a future.
At the moment, you don't have a diagnosis but I know the waiting is almost as painful as the diagnosis. I'm glad T is going with you to the appointment. I wish you all the best, and remember that even if the worst scenario comes true, there is still space for you.