Cheated a year ago. I don't know what to do.

I'll try to keep this short. First some backstory.. been with my guy for over four years. We were about 16 when we started dating. I had a bad home life and had to move in with him 2 1/2 years ago. If it wasn't for him I think I would be so much worse off right now. As every relationship does, we had some rocky moments... We were living in a small house with like 8 people whom were all his family. It was very stressful and of course he did not want to be there, neither did I but I didn't have a choice. He would run off to his cousins just about every single day to smoke pot and hang out.. At first I would go there with him but it was extremely boring and I'd rather put up with his annoying family so I could stay home and do what I wanted.
When he would leave I cried and begged him to just spend time with me. He acted like I was being clingy and annoying. After months of this I grew tired of trying and let him do his own thing. Our relationship was alright but I felt us drifting apart. I did a disgusting, awful thing that I told myself I would never ever do... I cheated. It was a 1 time thing but nonetheless it's awful and thinking of it makes me sick. What happened was as he showed me less and less attention I grew closer to this other guy, just as friends at first and I never ever thought anything else would happen. When it happened I was at my breaking point and honestly felt my bf did not care for me. At the time it seemed fun and like the thing to do... But as soon as it was over with I felt disgusted with myself.. How could I do that? Even if he had hurt me how could I betray him like that? And strangely enough a few days after the incident things drastically changed, but not because of that.
We had to move out of there and go stay with anther family member. He wasn't able to go to his cousins anymore so now he could actually devote attention to me... Which then I felt I didn't deserve because I did one of the worst things you can do to a person. I wanted to tell him because it physically hurts and makes me sick living with the fact that I did that. But I know if I tell him I will lose him for ever. We had this talk early on that cheating is no tolerance and he said if I ever hurt him like that he would immediately drop all contact with me. It is so hard living with this insane amount of guilt. Our relationship got better and now we get along amazing and he treats me really well .. Just making me feel more and more guilty. And I KNOW this is not an excuse but I just want to say I was molested and abused as a child for years and a lot of other traumatizing things had happened in my childhood. I truly was damaged and I know that people that have been abused are more likely to abuse others because that's what they were taught. I would never ever abuse someone but I feel cheating is somewhat like that because if the person finds out its going to rip their heart apart. But again I'm not trying to use that as an excuse because I knew it was wrong immoral and disgusting and I still did it.
Basically I don't know how I can live like this.. Telling him I love him and care for him more than anything else ( which I do I really really do) but in the back of my mind I know I crossed a line that I never should have. But I can't tell him I know 100% he will never speak to me again. Has anyone else been in a situation like this? If so please give me any advice. I know I'm an awful person so no need to rub that in my face.