I secretly helped my boyfriend's ex get her life back, and he still doesn't know

I've been dating my boyfriend for almost three years. Everyone thinks we're a perfect couple. We rarely argue in public, we travel together, and our friends constantly joke about how we're probably going to get married. But there's something I've never told him. About eight months ago, I discovered that he'd been secretly contacting his ex-girlfriend. At first, I didn't think much of it. They had dated years before we met, and he always described their breakup as mutual. But one night, while he was asleep, I noticed a message notification on his phone. It was from her. I know I shouldn't have opened it, but I did. The messages weren't romantic exactly. They were worse in a different way. He was telling her that he missed their old life, that he sometimes wondered what would have happened if they hadn't broken up, and that he wasn't sure he had made the right decision. I felt sick reading it. The next morning, I acted like nothing happened. A few days later, I found her on social media. I noticed she was going through a difficult time. She had lost her job, moved back in with her parents, and seemed to be struggling emotionally. I don't know what came over me, but I sent her a message. I told her I knew about the conversations and that she deserved to know what he was saying behind my back. She was surprisingly kind. She apologized for being involved and told me she had no intention of getting between us. We started talking occasionally. Eventually, I helped her find a job through someone I knew. I also encouraged her to apply for an apartment she had been looking at. A few months later, she was doing much better. Meanwhile, my boyfriend kept acting like everything was normal. He still occasionally checked her profile, but he never mentioned their conversations again. Then something happened that I wasn't expecting. His ex messaged me one evening and told me she wanted to thank me in person. She said that despite how uncomfortable the situation was, I had treated her better than he ever had. I cried after reading that message. Not because I felt sorry for her, but because I realized how much effort I had put into helping someone my boyfriend clearly hadn't completely moved on from. I still haven't confronted him. Sometimes I wonder whether I'm being immature by keeping this secret. Other times, I think he deserves to experience the same uncertainty he made me feel. The strangest part is that I don't hate his ex at all. I actually hope she finds someone wonderful. I'm just not sure I want to marry someone who made me feel like I was competing with a ghost.