hi - 18mnths is a VERY hard age - because of separation anxiety - here are some suggestions:
1. can the sitter come to your house to watch your daughter while you have class?
2. can you arrange for "playdates" with the sitter at times in between when you do stay there and play that way she gets used to be there with you and feels safe?
3. can you arrange for the sitter to come to your house in between times so she can get used to the sitter and start to build a relationship with her?
4. you may want to purchase 'special items" that can only be playd with at sitters house - that makes them special ya know?
5. arrange for the sitter to meet you for lunch or at the park that type thing - again to start to build a relationship...
Let us know how it goes!!
S. w.