I have to confess that two years ago I slept with a married woman...
How did you come accross this married woman?
She was a friend of my girlfriend and it happened one day, we both were drunk and we went to her house
If you knew she was married, you should not have slept with her, though it is a bit understandable given your drunken state. Still, you should not allow her to have any further affairs with you since it is highly unethical.
I didn't know it was wrong, I was drunk and women in general are attractive to me.
I understand that you were both drunk at the time and didn't realize that you were doing anything wrong.  I just mean that, from this point forward, you should not try to rekindle any physical passion with this woman, or accept advances from her.  It is unfair to both of your spouses.