Eigo is almost right except '대해' which means 'about' or 'for'. 

and '했던' is just 'have done' not necessarily done repeatedly. 

can a korean break this sentece down for me word by word please ?

내가 전에 했던 것에 대해 미안해
I know that it generaly means *sorry for what lve done * but l need the excat meaning please
Thank you ^_^

내가 이전에 했던 것들에 대해 미안하게 생각해

also since im a engilsh learner, im so sorry for explaining well in english. But at least ill try to make it sense to understand

the sentence u made is perfectly understanable but a part of '에 대해' is little bit unnatrual for me Because oftenly both for american and for korean are used to say like this, 'i feel so sorry~ or i think that~' 

therefore if i were u i would rather say like 미안하게 생각해

you know, everyone like to be uncertain and say indirectly

have a nice day