My boyfriend was unfaithful to his ex with me.

Hi, I'm a 26-year-old woman, and my boyfriend is 28. We've been together for five years and have been living together for four. We're even planning to get married and start a family soon. When I first met him, he had a bit of a "bad boy" reputation and received a lot of messages from other girls. I asked him if he was seeing anyone besides me, and he assured me he wasn't. My friends advised me not to worry too much about his interactions with other girls since we weren't officially in a relationship at that point, suggesting I should only care if he had a girlfriend. He claimed he didn't have one, so we continued seeing each other casually for a few months. Eventually, he opened up about wanting a caring girlfriend and expressed that he wouldn't need anyone else or even friends. After three months of casual dating, he asked me to be his girlfriend, and I happily accepted. A few days later, I overheard his friends jokingly ask him if he had broken up with his girlfriend. He laughed it off and said he would call them later. When I questioned him about it, he dismissed it as just a joke, and after meeting his friend, who confirmed his odd sense of humor, I didn't think much of it. Fast forward five years, we reminisced about how we first met and shared some funny memories. I decided to bring up that friend's comment again, and to my shock, he told me that it had been bothering him for years, and he was finally ready to tell me the truth. He revealed that he had a girlfriend the entire time we were seeing each other, even while we were intimate. I was devastated; it felt like a betrayal. He assured me he never cheated on me, spending time at the gym or out for coffee with friends, which were rare. He explained that his previous relationship was very toxic, and he wanted someone who cared for him, but he was too afraid to end it. He claimed they broke up just three days before we started dating, and during that time, he received constant calls and messages from her, which he dismissed as work-related. He said he stopped being intimate with her when he met me, but I'm struggling to believe that. I feel like my entire life has been a deception, and it's hard to accept that I was a second choice. TL;DR: I just found out that my boyfriend had a girlfriend while we were together, and it's shattered me.