I was in your boat in January this year, having been home with my daughter since birth. My daughter is also sociable but shy around new children so I was worried like you are.
But I am happy to say that it has been a fantastic experience for her and for me since day one. The key for us was that we prepared her for it well - we talked about it at home with her (what she would do when she got to school - playing, crafts, stories, building, etc., and that there would be other children to play with, and grown ups to play and look after her.)
Right from the first time I talked to her about preschool I made sure she understood that she was going to have fun, be with other grown ups but that I would not stay with her. I told her that I would drop her off and say a big goodbye, then come back at lunchtime to take her home again. I kept reiterating the come and collect you thing, as this seemed to be her main concern.
I found some books at the library about going to school which helped (Usborne have a good one), and also role played with her about what would happen when we got to school and how we would say goodbye. Then again how we would say hello when I came to get her. We also drive past the school a few times and pointed it out so that she was familiar, and found out the main teachers names so that she could learn them before time and they wouldn't feel like complete strangers.
I did what the teachers recommended - drop her off and say a good goodbye then leave straight away. Don't look back and show that you are worried or upset. She has been fine every day since, even at her new school that she started a few weeks back since we moved areas.
For you I would recommend keeping busy while she is at school and make the most of the free time. It will make you feel better about leaving her. Within a week or her starting I didn't know how I managed without that free time!! In my experience it has been fabulous for my daughter in building her confidence and social skills, trying new things. And it has given me some much needed time for myself.
Good luck!