My bf cheated on me 6 months into our relationship. I found out about it after 3 years. Can we move past it?

I, 19F have been with my bf, 19M for the past 3 years. We met online and have been in a long distance relationship since. We were each other's first love, so that explains the immense attachment and feelings. We had a pretty solid relationship despite the distance, usual fights here and there. However during 6 months of our dating, I began noticing him pulling away and acting distant. He told me its stress and since it was exam season, I had no reason to doubt him. But ever since then, he became really avoident and always "busy". I again thought nothing of it because I had no reason to doubt him of cheating and I too was busy with my studies. Fast forward to the 3 year mark, our relationship became rocky with only me putting in effort and feeling unloved. His same excuse of being "busy" now felt out. I had a sudden gut feeling that there was someone else and there was. He had an emotional affair with a girl he met online for 6 months. At first, I could not believe this was happening. I was pretty emotional and in shock. I asked him to explain but he refused to, claiming that it was years ago and had no significance. It was 10 months of me begging him to explain why he did it, but he refused to explain. During that time, I felt like I had lost my will to live (I am fine now and past it.) but overall, it was the most brutal time of my life and he was not there by my side. I realized that I had been struggling with grief as I refused to believe that any of this was real. Fast forward to 3 months, he finally explained why he did it and recognized the effect it had on me. His main reason was "he was too immature to think as he was 16". He wanted to work on the relationship and I gave him a chance because I still had a little hope. Ever since then, he had been very very honest and a little too loving and caring. He has allowed me to express my anger and has been supportive. But even after all that, I still feel the betrayal and anger of his cheating. He has changed, no doubt about it, but every time I see him, I see how he replaced me and left me all alone when I needed him. I want to know if this is something we can move past this or if this is a serious and valid reason to end this relationship. PS: we met last January and he was my first kiss