N.B.
Routine, routine, routine. Ask her teachers how she does once there? What do they do to change her bad funk? Maybe they do the same exact thing each time? They may already have a set specific routine (Like directing her to certain area of the room or a task or to go to Miss So & So (another teacher in the room). If so, this may be what she needs from you in the time leading up to it.
Most preschools, or even a "play based" daycare such as my own, follows a routine and schedule of some sort. This lessens anxiety in kids, so they know what to expect and in what order their day will mostly go. Many children need this to feel safe and secure in their place in this new adventure of their lives...to know where they fit and whats happening with it all.
I would try a specific routine with her in the time leading up to preschool?
Maybe just do the same thing in the car on the way there if that last bit is where she falters?do it exactly the same each day...."here we go.....stop for the stop sign.....wave to the kids waiting for the bus.....theres the library...., etc" Whatever is on your route that she knows, so there is an order. Then arriving at the parking lot is expected along the way, rather than like a bomb going off at the end.
I had a daycare client who had to do this for quite a while..and they had fun with it. They would go past the river, and in the winter the birds lined up along the ice along the edge...and the Mom would say "here comes the river, lets see if there are any birds so we can tell Nikki and our friends". He would come in the door saying "The birds were waiting at their bus stop on the ice all lined up!" He turned it into something to be excited to share with us on the short drive here. (His anxiety was a specific reason..Dad had just deployed to Iraq...little man was 2...and he didn't like Mom leaving his sight...afterall, Dad left and he didn't understand why...what might happen if Mom left?). It became the NEW routine.
Good Luck!