He lied

My boyfriend lied to me to my face multiple times about something that had happened in the past. He was my first everything so it's crucial for him to have been honest with me. Long story short; before we started dating, he was in a relationship with one of his good friend's sister and they lasted for not even 6 months. Fast forward 8 months later and he and I started talking and two months later officially dating. I of course asked about his past relationships and he said after he broke up with his ex's that he had and wanted nothing to do with them. Little did I know that he was having sex quite frequently with his ex throughout the time that they had broken up during that 8 month span. He was actually having sex with her up until we became official. I started having sex with him soon after we became official, and like I said before he was my first everything. So almost a year later finally picked up on all of the pieces and found out that he wasn't honest with me. I don't know if I'm more upset over the fact that I have him plenty of chances to just be honest with me and he lied or the fact that I quickly went from not ever being in a relationship to having sex with a guy who was sleeping with another girl when we were talking. Idk, I'm overthinking this, I just wanted to let it all out. Just to make it clear he did stop having sex with her when we became official.