My 4 year old just started preschool for the first time, ever. She doesn't even go to a babysitter, just me and close family. Needless to say, we had a very rough time. Her teacher shortened her day from 3 hours to an hour. As soon as I dropped her off the teacher would set the timer w her. When I picked her up we reinforced "mommy comes back, see mommy came at the end of the timer!" We gradually increased her stay and were consistent w the timer. Within a week she was a different kid! Now she runs out the door to go to school.
She also made friends w another little girl. We schedule playdates outside of school. GL and pm if you need anything.
My guess is this will pass- stay strong!