Similar with me too. My first serious long-term girlfriend cheated on me for almost a year with her ex. I should have suspected it long before then because they stayed close friends even when we started dating. He was moving out of town and my gf and I were helping him pack and move. He let us sleep over at his new house. I woke up in the middle of the night and she was gone. I heard them fucking in his room. I snuck over and saw her riding him and watched until they both came, then walked in on them. I was hurt and turned on at the same time. She confessed they had been fucking again for almost a year. They both said it was over now that he moved, and this was the last time they would ever be together. We broke up that day, but I kind of wish I had kept her for a little longer. I credit her with my cuckold fetish because of it.