I was quite worried it was so bad he/she didn't want to tell me.
この場合、次のような言い方ができますよ。
ーI was quite worried it was so bad he/she didn't want to tell me.
「悪すぎて私に言いたくなかったのかとかなり心配した」
例：
I have a relative who was hospitalized, but they didn't tell me the nature of their illness. I was quite worried it was so bad he/she didn't want to tell me.
「入院している親戚がいるが、病状を教えてくれなかった。（病状が）悪すぎて私に言いたくなかったのかとかなり心配した」
nature of their illness で「病状」
ご参考まで！
カナダ 
