Lindsay,
I could have wrote this myself! I'm 24 & experience everything you've listed here with the exception of extreme nausea. I just posted a thread called "weakness" yesterday as a matter of fact. I've been tested for MS, negative. I've had blood tests, a brain MRI, worn a heart monitor for three Weeks, seen a neurologist...I have no indefinite diagnosis. Anxiety? I have a hard time wrapping my head around it but it's the only logical diagnosis.
What have you/are you doing for your anxiety? I currently see a therapist that uses CBT technique once a week. Many, many people have received a great benefit from CBT therapy. Another would be medications. I can't help to much there since I've never taken medication for my anxiety. I've had anxiety since 12 years old :-( only lately pretty severe.
Sounds like you've been checked out medically pretty throughly. I would try to accept that it's anxiety related & then address your anxiety & you should start to see a difference. It's a vicious circle that takes lots of time & effort but there is a cure. It's uncomfortable to have anxiety but it's not life threatening. Hang in there. Keep posting