She was supposed to come to my party, but she couldn't make it, so I was sad.
She was planning to come to my party, but she didn't end up coming, so I was disappointed.
この場合、次のような言い方ができますよ。
ーShe was supposed to come to my party, but she couldn't make it, so I was sad.
「彼女はパーティーに来るはずだったのに、来れなくなって、私は悲しかった」
She was supposed to come to ... で「彼女は…に来るはずだった」と言えます。
ーShe was planning to come to my party, but she didn't end up coming, so I was disappointed.
「彼女はパーティに来る予定だったのに来れなくなって、私はがっかりした」
She was planning to come to ...「彼女は…に来る予定だった」を使っても表現できます。
ご参考まで！
She was supposed to come but she couldn't so I'm sad.
彼女が来るはずだったのに来れなくなってしまって悲しい
- She was supposed to come but she couldn't so I'm sad.
- I'm sad that she was supposed to come but couldn't.
- I'm disappointed in the fact that she couldn't come even though she originally was supposed to.
彼女？ girlfriend?
- My girlfriend was supposed to come but something came up so I'm a little disappointed.
- I'm sad that my girlfriend wasn't able to come even though she was supposed to.
カナダ 
アメリカ合衆国 
