過去我真的很快樂 的英文

I was really happy. 就可以了，用過去式就表示過去了，不必再加 before。
如果要更強調一點，還可以用
I did be really happy. 不過比較少人這麼說，這是語氣非常強烈的用法，表示過去是真的、真的、真的很快樂，不是假的。

過去我真的很快樂。
I was really happy in the past.
I was really happy before.
I used to be really happy in the past.
I used to be really happy before.

past 是「過去」，before 是「以前」。
in the past 是「(在)過去.......」。

I used to be really happy.
這是我自己翻的

I used to be very happy.
I was once happiness itself.