Haunted by thoughts of them together

My husband, let's call him Mark, and I started as a no strings attached, friends with benefits kind of agreement. We we're just fooling around but no sex, I felt/ feel like no sex before love. After a few months we fell for each other. The first thing Mark tells me when we decide to move forward as a real relationship is during the months we were fooling around he slept with my roommate twice. My roommate, let's call her Nicole, had been my friend for close to 10 years. I flew out to Toronto to visit her when she lived there, invited her to my families home for Christmas because her family situation wasn't as good as my own, poured myself into her and her friendship for years. I gushed about Mike to her for months, I talked about how I wondered if he was sleeping with other people, if he cared about me, and she was completely silent. After years of friendship she lied to my face for months and months. She knew I cared for him, how I felt about sex and she chose to sleep with him twice. Now, two years later the thought of them sleeping together still haunts me. I can't be around her. I can't even have friends who are friends with her. Even her photo gives me a physical response. My heart starts beating out of my chest, I get hot, shaky and uncomfortable. My husband knows he really messed up. He tells me all the time. He's been patient and kind with me and he's truly changed. But even with that admittance of guilt and years of working hard to make me feel like he needs to work for my forgiveness, I can't shake those haunting images and my sadness/ anger towards Nicole. What should I do?