다짐했던 << 다짐하다 + 었던 다짐하다 : to resove 었던 : had p.p. 10대 : teenage 대 means "range of age" 다짐했던 나 : Me who had resolved to do something + 10대의 나 : Me who was in teenage 다짐했던 10대의 나 : Me who was in teenage and had resolved to do something The whole setence is too long and complicated. 내가 할 말은 아끼지 않는 사람이 된다고 다짐했던 10대의 나를 Me who was in teenage and had resolved to become a man of not refraining from saying one's picece 나는 그냥 놓아주지는 않았고 I didn't just release and ...

10대의 means In my teenage years Age 10 - 19 20대의 means In my twenties