My boyfriend was unfaithful to his previous girlfriend and kept it a secret. I'm not sure how to feel about this.

I'm in a new relationship that has been going on for a couple of months. On our first date, he was very open with me and wanted to address any 'red flags.' He admitted to cheating on his ex-girlfriend but only revealed it to her after they broke up. Initially, I appreciated his honesty, but as someone who struggles with trust issues, this information is difficult for me to digest. Since then, he's shared more about the situation, and what I've learned is concerning. He was just 21 at that time, and while I understand that young people can make mistakes, he had been in a relationship with his girlfriend for roughly two to three years. He cheated with one of her closest friends, who lived with them. It started when they were alone together, had a few drinks, and things escalated from there. This incident happened a few more times because they wanted to explore if there were any feelings between them, which they ultimately decided there weren’t. As far as I know, they continued living together for a while after that. He chose not to tell his girlfriend about the cheating, fully aware of the likely consequences. He kept it a secret until he was 27, claiming he 'struggled with guilt for years' but assured me that he would never cheat again because he cannot bear that guilt. However, I recently found out that he only told his ex about the affair because she confessed to cheating on him and had developed feelings for another guy (who she is currently with). At that moment, he still didn't come clean until he sensed she was torn between the two of them and might choose to give their relationship another shot. He felt it was necessary to finally be honest with her, resulting in her ending things, which I completely understand. It seems that both of them contributed to a messy and toxic situation. She was almost ten years older and deeply entrenched in their relationship, while he was much younger. At least she was honest about her feelings, whereas he kept his secret for years, and that's something I struggle to reconcile. I also get the impression that he sees himself as the victim in their relationship. While it's true she left him for someone else, I can't help but think that if he had been upfront, she might have made a different choice sooner. He expresses frustration at her for not giving him a chance to fix things, which feels a bit like an excuse to me. So here I am, with a boyfriend who cheated when he was younger and kept it hidden for years. Now at 29, I don't want to hold his past mistakes against him, but I'm genuinely concerned, and it weighs on me. I've tried to communicate that this is challenging for me, but he seems to think he is being punished for something he did in his early 20s. While I can acknowledge that he made a mistake, his attitude surrounding it worries me. I really like him and want to give him a chance, and I believe him when he says he wouldn’t cheat again. But I can’t shake the fear that it will impact me in the future.