I have kept this in for so long, but he found out

I have been with my boyfriend for 6 years. We have lived together for 5 years. He is the love of my life, my world, my rock, my solemate and my everything. I love him, I am in love with him. But 5 years ago I made a huge mistake. One that I have regretted every day sence. I had sex with another man. It was his best friend. I was drinking, young and stupid. I had too much to drink, went to lay down to stop the spinning. His best friend came in to see if I was ok. He gave me a kiss on the cheek. Then one on the lips. Then started to touch me. One thing happens right after the other and next thing I knew he was on top of me. At that moment had realized that it was a huge mistake. And told him to get off of me, he did not. Then I tried to push him off of me I could not get him off of me. I have kept this from my boyfriend for so long. He found out 3 days ago. But now blames it on me. He says he loves me. And I know he does. He said I should have told him. And I should have. But I felt like a **** like I did something wrong. Like if I told him, even though I was not a willing participant that he would leave me right then and there. And I have wanted to tell him every day sence it happened. But I just could not find the right time to do it. When he found out. He was so mad at me for keeping it in for so long and that I did not feel like I could tell him. After so long I just left it alone and figured that I would be able to live my life and move on. Then it comes out. Now he does not trust me at all. And says that I have to prove to him that I love him. And that he is the only one for me. He is all I want and all I will ever want. How to I prove to him that I love him and that he is the only man for me. How do I get him to trust me again?