I wish you all the very best with your appointment.
You may find this link helpful, I know I did as I recently saw a psychiatrist about my anxiety.
http://www.mayoclinic.com/health/gen...ur-appointment
It has a list on there about questions your doctor may ask so it is worth considering those. Generally,t hey want to know about the experience for you and how you are affected, how long it has been going on and if it is triggered by anything.
There's also a list of things to consider in the lead up to the appointment.
What they suggest will vary so I can't comment on that but if you feel you've been trying diet/exercise and it hasn't alleviated your symptoms you should let them know what (and anything else you have already tried, for that matter).
As for phrasing it, take a deep breath, and just talk about what you're concerned about. I sat down with my psychiatrist and said something like 'I'm experiencing a lot of anxiety and it's getting very difficult to deal with', and from there we explored it some more.