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って英語でなんて言うの？

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！

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.