Separation anxiety

Our 20 month old is really struggling. If we leave the room and go upstairs she loses her mind. If family come over to play with her, she won’t go anywhere near them. Even people who were once her favourites. This has been an on and off problem since she was about 10 months and she’s had phases where she’s warmer to people but we’ve been in this bad phase for several months. What can I do? Is anyone else going through this? She doesn’t start nursery school until fall and she’s not in daycare.