せっかく良い友達って英語でなんて言うの？
せっかく良い友達が出来たと思ったのに。は、どう言えば良いでしょうか？良い友人が出来て喜んでいたら、実は良くない人でガッカリという状況の時です。
回答
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I thought I had made a good friend.
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I thought she was a good friend to me.
この場合、次のような言い方ができますよ。
ーI thought I had made a good friend.
「良い友達ができたと思ったのに」
ーI thought she was a good friend to me.
「彼女は私の良い友達だと思ったのに」
例：
I thought I had made a good friend, but it turns out she wasn't such a good friend after all.
「良い友達ができたと思ったのに、やっぱりそんな良い友達ではなかった」
to turn out で「〜であることがわかる」
after all で「結局・やっぱり」
ご参考まで！
回答
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I was so happy that I had made a new friend, but was really disappointed to find out that he/she wasn't as great of a person as I had thought.
ご質問ありがとうございます。
"I was so happy that I had made a new friend,"＝「新しい友達が出来てとても嬉しかった、」
"but was really disappointed to find out that"＝「でも～だと知ってとてもガッカリした」
"he/she wasn't as great of a person as I had thought."＝「彼・彼女は私が思っていたほど良い人では無かった（ということを）。」
ご参考までに。
カナダ 
アメリカ合衆国 
