Can people really change or am I being hopeless

In the beginning of meeting my current boyfriend I was 16 and he was 18. He had an online girlfriend who he swore he loved and they were both into like kinky sex stuff. They wanted me to join their polygamous relationship (at least he did) but I refused because I wanted him for myself. We still started doing stuff together, he was the first person I was ever intimate with. He would sometimes not want to do things because he was reminded he had an online girlfriend and it would crush me. Looking back now I know I should've left as soon as I knew he had a girlfriend but I was young and he was he only guy I ever did stuff with. One day we meet up because I told him i wanted him to take my virginity. We had sex and it was traumatizing. I wasn't ready, I was barely comfortable letting him see me with my pants off at that time so when we had sex it was so traumatic. I felt like I had given up my virginity to a guy who didn't care for me like I wanted him to. He then told his girlfriend we had sex and they eventually broke up because of it. He would constantly question if dating me would be a good idea after that, partly because I was still in high school and partly because I don't think he wanted to settle. When we did start to date he made it clear that this was an open relationship, and that if either of us want to fuck someone else we could. Then several months after that he asks if I'd want to be in a 3-some with his a different ex girlfriend. This other ex girlfriend kept asking him for sex. When I got angry he played it off and kind of joked around in a kind of "I was just asking, jeez" kind of way. I think that happened within 6 months of us dating. After that I feel like he has changed. He no longer talks like he used to, he's now 21 and has started going back to college. When I bring up how he treated me he cringes because he genuinely doesn't like how he acted. And I have nothing but love for him now, he treats me so well. He supports me and care for me and my concerns. He's literally the perfect boyfriend to me. I just can't help but wonder how much can someone really change. I've always believed people don't change. But his actions have told me otherwise he's a completely different man than he was 3 years ago. I don't know what to think