If the relationship gets broken just by that, it won't last for long anyway.
He was just not that into you if that was the reason why he left you.
If that was the reason why he left you, it wouldn't last for long anyway.
「そこまでだ」は「(どうせ)長続きしない」という意味だと解釈して、回答を作りました。
relationship 関係
break 壊れる
last for long 長く続く
is into someone 誰かを大好き
he left you 彼があなたと別れる(あなたから去る)
If the relationship gets broken just by that, it won't last for long anyway.
それだけで関係が崩れるなら、どうせ長くは持たないよ。
He was just not that into you if that was the reason why he left you.
それが彼が別れた理由だったら対して君のことを好きじゃなかったんだよ。
If that was the reason why he left you, it wouldn't last for long anyway.
それが彼が別れた理由だったら、どうせ長続きしていないよ。