It's because OO is difficult that it's worth doing.
It's because OO is difficult that it worth doing.
ご質問ありがとうございます。
ニュアンス的に2番目と3番目の回答はAnna様が伝わりたいこととぴったりです。OOのところに動詞のING形を入れることになります。
例文:It's because learning Japanese is difficult that it's worth doing it.
ご参考いただければ幸いです。