I slept with my best friend's husband

I did something terrible i slept with my best friend's husband, i don't know how it happened, i went to her house for a visit, she was not there, but her husband said that she'll come back soon, and offered me to stay and wait for her. Then waht happened next was so quick, that i don't know why and how it happened.     i feel really guilty and ashamed, she is my best friend, and her husband (the one i slept with) is like a brother to me, he helped me alot before, during my life crises.   it was a one time (night) stand, but i hate myself for doing it. what shal i do?