Play music when you ride it helps me alot
Keep her brain working. Don't make it boaring for her. Lots of transition work lots of changing direction.
POM's in the ears and lots of different exercises to keep her mind busy. More importantly make sure you are completely relaxed so she doesn't pick up on any nervousness :)
Keep her working hard mentally with circles, transitions etc. Don't stay on the rail for more than half a lap. She won't have time to think about her friends.
Mine was the same way. I would lunge her briefly then walk in each direction before mounting all while talking to her. I would also make sure I was not causing the problem by being tense myself. Deep breaths; they feel them. On top of the previous advice of patterns, transitions and keeping them occupied.
Lots of patterns, halts, half halts, changing directions, anything to keep her mind occupied and not worrying about the fact that she's alone. Best of luck! :)
You could try giving her "chill" (or another brand like it) to calm her down until she gets used to it. It's all natural.