L.T.
I feel for you. Sleep deprivation is a awful and we hate to see our little ones miserable.
A few questions came to mind as I read your request:
Has there been a change in her bedtime routine? Do you have a bedtime routine that will help her learn to expect that bedtime is near?
Did this behavior change correspond with putting her in a toddler bed? If so, maybe she isn't ready for this sort of change in her surroundings.
You mentioned she lays behind the door and cries. Is the door completely closed? If so, maybe she is scared by that.
Does she have a snack before bedtime? If I skip the evening snack for any reason, my daughter (also 19 months) will wake up in the middle of the night.
Good luck!