Gain back the trust

I am more in love with a guy than I had ever been in my entire life. We have spent an amazing (close to) 3 years together, and there wasn’t a day that I did not want to be with him. We lived next door to each other in college, so going home for the summers was always difficult because we lived far away. Last summer, I met a guy on vacation and kissed him. It meant absolutely nothing, and I truly felt that me and my guy had ended what we had, since we had an extremely rocky summer. When I got home and moved back to school, I soon realized it was a mistake and despite everything that happened over the summer, he did still love me. He found out about the guy I kissed, and was deeply upset. He did however, take me back and apologized for becoming mad at me. This past weekend, we had an amazing time together, as always. However, yesterday he broke up with me. He explained that I did something Saturday night which made him lose trust in me. I spent the entire night with him, and therefore I am totally confused as to what I did. He will not tell me because he says I should figure it out on my own. Help! He said he will still be my friend, but I am dying inside. I miss kissing his lips already and calling him mine and it has only been a few days. I would NEVER do anything to hurt him, but somehow I still managed to. I want to be back in his life, but how can I get there if he says he doesn’t trust me anymore? He admits he still is very much in love with me, but you cannot have a relationship without trust. I didn’t do anything wrong… How do I get him back?