YOU SAID:
Being with him was my happiness.
INTO JAPANESE
彼と一緒にいることが私の幸せでした。
BACK INTO ENGLISH
It was my happiness to be with him.
INTO JAPANESE
彼と一緒にいられたことは私の幸せでした。
BACK INTO ENGLISH
It was my happiness to be with him.
That didn't even make that much sense in English.