HELP I'm sinking.

Today was a nightmare!! I found out my husband of 20 years had a "fling" He confessed because the other womans boyfriend was going to tell me. I am so confused. He wants to make it work. It was just sex, I don't know why I did it,She didn't take no for an answer, I caved. It meant nothing,.... He is relieved, because the guilt was getting to him..So he says. I don't know what to think. I thought this was the man I was destin to spend my life with. Do I try to save my marriage/family. I don't want to be betrayed again. I need guidance. I'm so lost, ashamed of what he did. Dear Lord, Please guild me in the right direction.