I did something bad. Now I'm in an awkward situation with a housemate and his gf?

I’m embarrassed writing this. My housemate and I kissed, but he is in a relationship, and me and her have actually become friends.
we decided not to do anything about it and not to tell her.
now when she comes round i am overcome with guilt and he babysits us when we’re alone together. i cannot continue like this, and need moral guidance from you guys at fluther.