I wanted to break up with him, but I had trouble telling him for the longest time because I thought he would be quite sad.
この場合、次のような言い方ができますよ。
ーI wanted to break up with him, but I had trouble telling him for the longest time because I thought he would be quite sad.
「彼と別れたかったけど、彼が悲しむと思ってずっと言えないでいた」
to break up with ...「…と別れる」
to have trouble telling someone「人に言うのに苦労する」
ご参考まで!