Fucked boyfriend's dad

I have a major issue. Last night I had sex with my boyfriend's 54 year old dad. I've always had a thing for older men and something about his dad just turned me on. I know I made a terrible mistake and I still haven't come to grasps with it. I couldn't fall asleep last night, knowing I ruined a truly faithful marriage and could possibly ruin their family. Having gone out with my boyfriend for 3 years, I have the respect to tell him what happened but don't know how to go about doing it... Should I tell him? If so, what should I say? I'm really torn apart about what to do... I've never felt this shitty about myself. S