Wait... why do I feel like I'm the only one who doesn't get it..?
Shion save the dance.. thats correct.. but she's not jealous.. she didn't let go of Kiki because of jealousy
Rather
For me she can't bear the feeling and nostalgia of how dancing with Kiki made her feel like old times, the feeling how Kiki leading her.. and so as desired take over. She pushed Kiki away. Because she just can't bear it.
Rather hating her.. I feel pity for both shion and mona.. they could have won.. it weren't for shion saving Kiki.
For me it felt she realised Kiki was leading her which means Shion herself was trying to lead and upon realising being with Kiki would drag her back to "suit Kiki", she pushed her away showing she knows what is best for Kiki even if she does not want to which we see by how distraught she was.
She compared Kiki before the dance, wearing her dress, to last year, where she had a suit; and, liked her in a dress more, but her reason is important: "What a difference it makes to have an outfit you actually want to wear." Her characterisation, character arc, and character development have all focused on Shion wanting Kiki to be able to be herself, but his a teenager and as such does not have a lot of life experiences to draw on which I think people are quick to ignore. She loves Kiki, but understands she is not the person Kiki needs even if she does not know how to handle that. But here we see her immediately realise she is dragging Kiki back into a lie and shove her towards the person who can make Kiki's dream come true at the cost of her own feelings. That is hard for anyone, but especially teenagers.
