Have you tried counselling? Real life can't compete with the excitement of affairs, that's why people have them, as an escape from reality. In that relationship you're not all the things you've fallen into being with your dp - the parent, the one who irons the pants, the one who cleans the toilets or whatever - you're just the sexually desirable person that maybe you don't feel like at home. It must be really hard to move from having those 2 separate lives back to a healthy, functioning relationship which satisfies on all fronts.
There might be all sorts of issues:
Does she feel attractive to you? Do you find her sexy, and does she confidently know that, or does she doubt it? Does she feel self conscious, old or mumsy? With an affair partner, you can pretend to be someone else, someone confident and sexy. Your partner knows you're just you.
Is she still sexually attracted to you? How was your sex life before the affair? Were there issues which haven't been addressed for a long time?
Are there issues such as guilt or trust issues, anger or hesitation on one or both sides after the affair?
Are there deeper issues than the sex? Does she love you? Do her actions show warmth and kindness to you? Does she seem to want to make you happy, or do you feel it's more that she just wants you to stay?
Presumably, if she has cut all contact with OM and the relationship is otherwise on track now then it's because you're BOTH prioritising each other's needs and feelings, she's remorseful, and you've both seriously re-committed to being each other's priority and really working on the relationship - is this what's happening? Or do her actions suggest she's still not in it 100? I'm talking about honesty, openness about feelings, transparency about where she's going etc to build trust again, is she doing everything she can?
Affairs are difficult as fuck to get over, even if both parties are trying their best. If you're both desperate for this relationship to work I'd really recommend investing in some sessions with relate, and both you should read Shirley Glass "Not just friends". Good luck.