When you did something wrong the expressions 미안해요 (mianhaeyo) and 죄송해요 (chwesonghaeyo) come in handy to say sorry. When you want to admit you did something wrong or if you want to acknowledge that something unfortunate happened and it was your fault the expression would be 잘 못 했어요 (jal mot haesseoyo). This means I was wrong or sorry it was my fault.
This expression is very common and you can almost always hear it on dramas. It is normally said the casual way, 잘 못 했어 (jal mot haesseo). Literally it means ‘I am not good’.