I cheated because I thought he would cheat on me. Now I still feel terrible. What should I do?

I have been with my boyfriend for over a year, and we even moved in together. I was and am so in love with him that it scares me to death. I am always preoccupied with losing him to another woman. I still feel it will happen. He is the kind of guy that attracts women of all ages and I am not twenty any more, and I know men like to be with younger women. It was making me so sick thinking about it that I went out with someone else. I wanted to feel that I too was attractive to other men, but I still feel terrrible. Maybe I need to let my boyfriend go now, to save myself future heartache. It will destroy me when he leaves and I know that no other man will take his place. I need to be with someone that doesn't make me feel so scared.