Honorary Middle Child

This would be an oldest sibling, who was ignored like a typical middle sibling but probably due to their parents splitting and having children with their new partners

"What's their problem?"
"They're upset because they're being ignored like a middle sibling."
"Oh they must be an honorary middle child."

Disney hope

When you really hope something will magically change for the better, usually in a romantic way

I had disney hope that my ex would change their mind and come back.