If I knew that our relationship was going to end, I would've treated you so much better.
Had I known we were going our separate ways, I would've been a better girlfriend.
①If I knew that our relationship was going to end, I would've treated you so much better.
「（私たちの関係が）終わることを知っていたら、あなたのことをもっと大事にしていたのに。」
If I knew→もし知っていたら・わかっていたら
our relationship was going to end→私たちの関係が終わる
I would've treated you so much better→あなたをもっと大事にしていたのに
②Had I known we were going our separate ways, I would've tried harder to be a better girlfriend.
「もし別れることを知っていたら、もっといい彼女でいるためにもっと努力したのに（もっとがんばったのに）。」
had I known→If I knewと同じ意味です。
go our separate ways→別れる、別々の道を行く
I would've tried harder to be a better girlfriend→もっといい彼女でいるために、もっと努力したのに（がんばったのに）
「もっとあなたの笑顔が見たかった」
→I wish I could've seen your smile more.
少しでもお役に立てれば幸いです。
日本 
