Here is my answer.
'I should have never met you, because it will hurt so damn much, when I need to go back.'
(In a formal way) 당신을 만나지 말걸 그랬어요, 왜냐면 제가 돌아가야 될때 많이 힘들테니까요.
(In a friendly way) 널 만나지 말걸 그랬어, 왜냐면 내가 돌아가야 될때 많이 힘들테니까.
As you may know, you speak 'in a formal way' when talking to someone older than you or those who are not your frineds (ex. Strangers, people you know in business, or etc.)
And you speak 'in a friendly way' when you are talking to your friends.
But you have to be careful when you start to speak 'in a friendly way' because your friends who are older than you may feel bad if you speak in that way without asking them before.
So it is always good to ask them if you are allowed to speak in a friendly way.
Hope it helps! C: