I remember our first trip to DL about 5 years ago. My anxiety was through the roof, I was in a bad way. It was also DD's first trip out of Australia, and my first o/s trip as an adult, IYKWIM.
Fast forward all these years, and this will be the 4th trip, I'm still a bit nervous, but it's easier. All of us here can help with what you can expect.
From memory your DD is in a wheelchair? The US is a lot more disability and wheel friendly than Australia is. My DD has needed to use a wheelchair through knee injury and it is a lot easier, to the point that you don't even think of having a wheelchair or how to get around with it.
How did I get through my nerves - I don't know that I ever have. Being prepared, I have a full intinerary in a display folder that gets updated with all our print outs. I Google street view driving and walking. I have our passports and everything else ready to go. I also drank lots of tea!
Meditation also worked for me, each night before bed I meditated so I could get a good nights sleep.