First off: Congratulations for making the call!
Your post made me smile, not for the way you were feeling, it's just that I felt a lot like you described. I had my letter in hand from my therapist. She had recommended a doctor that she works with. No problem right?
But still........ What do I say? How do I say it? Does anyone else actually do this? Will they even know what I'm talking about?

All of this stuff ran through my mind for a few days before I finally called.
When I did, I was on my way home from work and pulled over in a grocery store parking lot to call. I had finally waited long enough that I couldn't stand it anymore, so I sat there with my phone in my lap for what felt like another eternity talking to the stupid thing like it was going to answer me back.
Finally though, I hit send on the phone and called. I swear my voice had gone all squeaky by the time I said what I was calling for. The receptionist was like "Oh yes! Of course we do that, can you come in Wednesday?" Just like that, matter of fact....
Turns out they already had my letter. It had been emailed over by my therapist by the time I got to my appointment so I didn't even need to bring it with me.
Lots of nerves for no reason.