せっかく

The word せっかく can be used with nouns or full sentences to express a situation where the speaker put a great deal of effort into something, and therefore wants to utilize whatever resulted from it because of that effort (or express regret because they can't utilize it). It can also describe a scenario that is rare and that should therefore be cherished or otherwise used.

せっかくケーキを焼いたのに、誰も食べてくれなかったから、がっかりした。
Even though I went through the trouble of making a cake, no one ate it, so I was disappointed.

せっかく日本に行ったのに、ホームステイが出来なくて、残念だった。
Even though I got to go to Japan, I couldn't do a homestay. It was too bad.

せっかくの野球の試合が雨で中止になってしまった。
The long-awaited baseball match was called off because of rain.

Ａ：今夜、私の家に食事にいらっしゃいませんか。
Ｂ：ありがとうございます。でも、せっかくですが、今日は仕事が忙しくて・・・。
A: Tonight, would you like to come to my house for dinner?
B: Thank you very much. I know you went to a lot of trouble for it, but today is busy for me...