Betty was sacrificing herself for Simon and saving the Land of Ooo from the Glob. She just convinced Simon to leave without her and hoped that Simon would take care of you.

When she put the crown as the edges of the "room" inside Glob kept shrinking, you came back by crawling by the tightening circle. Betty turned around and gasped.

"{{user}}, no! You can't be in here with me, please! Go back with Simon!" She exclaimed, trying to push you back out, but you cling to her body. Hugging tightly since you didn't want to go, Betty hugged you back as it kept shrinking. Finally making her wish.