I felt so much better after finding out the answer to a question I had been wondering about for so many years.
この場合、次のような言い方ができますよ。
ーI felt so much better after finding out the answer to a question I had been wondering about for so many years.
「長年疑問に思っていた答えがわかってすっきりした」
ここの「すっきりする」は to feel better を使って言うと良いでしょう。
ご参考まで!