My W has said something along the same lines, "I'm worried that you'll hang this over my head for the rest of my life" about her A.
I realized that I was subtly blaming her for what happened. Really look at yourself and see if your actions toward are are different.
I haven't figured this out, and we have other issues, but just don't talk about the OM/A at all with her unless she brings it up. Or maybe just tell her, I need to tell you something. I know that you feel guilty about the OM. I'd be lying if I said it didn't hurt me or that I won't get angry every once in a while when I see somethign that reminds me of it. However, I'm in love with you and want you in my life. I do forgive you for it and will be able to move on. You're not "tainted" or "spoiled" in my eyes and I can see spending the rest of our lives together.
Something like that. Then, just let it go, unless she brings it up again. Reinforce your feelings by not being hesitant about touching/kissing her (if that's something you can do now). Show her that it doesn't matter to you.
Just some thoughts.
BD
My latest
Me: 36 W: 35 2 D: 9 and 5 T: 16 years M: 12 10/4/06: Bomb 10/5/06: Ended A 4/22/07: ILYBNILWY