Whew! What an emotional roller coaster. Were I you, I would continue to use caution in the relationship. Having also had two wives I know how vengeful a scorned woman can be. Sarah's trust has been stretched if not broken. Re-establishing that trust may be impossible. Midway through my first marriage my wife had a one-night stand. I found about it from her girlfriends. I forgave her it took years for me to trust her again.
I got really drunk one night and asked the guy i wanted if he would sleep with my gf he said damn right. My gf was really into him so she was sitting on his knee in a flash. They just looked at me and both together said well good bye. I left the room and pretend to leave the flat. I stood outside the lounge stroking my cock she loved shagging him and I can't blame her. He was really big and looked so hard. She sucked him then rode him for what seemed like ages. I had came within minutes. It took 2 years to get her to admit it. Apnd about another year before we got him to do a 3sum with us.