ㄴ/은 데 Ending
How is this used? I've looked it up and everything says its the equivalent to and or but in English, but I've heard people say 하지만 after it.. And I heard in a video someone ended with 는데 and that was the end of their sentence, then the person they were talking to answered them.. Does it have more than 1 meaning and how is it used? Try to make it as simple as possible, every other explanation I can't seem to understand. :L Thank you! ^o^

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Would informal speaking be in songs? I wouldn't think so but I can't figure out why it would be in this song where it is. Here are the lyrics:

달라 달라 나는 좀
해보고 싶은 그냥 하고 말지
고민 고민 하다가
어른이 되면 후회 많을 텐데

It's from the song Happiness by Red Velvet. And in the English translation the last line is:

You’ll have a lot of regrets when you’re an adult

then the song continues with:

Yesterday, today, tomorrow too
My diary is full of finding happiness

So I'm confused as to why ㄴ데 was added there. Is it used because the girl sings right after?  Thanks :)

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Hi Bailey, Maria's answer is very good. I just want to add something. In informal speaking, we often finish our sentence with "~데" instead of "~했어". For example, "나도 거기 갔었는데."="나도 거기 갔었어."="I've been there too." Or "걔는 그때 한국 갔는데."="걔는 그때 한국 갔었어."="he/she went to Korea at that time."