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！
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。
・「I thought I finally made a good friend.」
（意味)せっかく良い友達が出来たと思ったのに。
<例文>I thought I finally made a good friend. I'm just so disappointed.
<訳>せっかく良い友達が出来たと思ったのに。すごく残念です。
ご参考になれば幸いです。
カナダ 
アメリカ合衆国 
