How can I get over lying to my best friend?

My best friend and I have been best friends for 17 years (since we were 11). We have been through a lot together. Last year I decided to become celibate for religious and personal reasons. My boyfriend couldn't deal at the time so we broke up. My best friend supported me through it all and hates him because of some of the things he said to and about me after the break up. Well a few months later he came back,apologized, and decided that he wanted to become celibate too. It didn't work out for he and I being together and trying to practice celibacy so we separated again. She knew about it and again was super supportive. What she doesn't know is that since September he and I have been back together and have broken our celibacy vow together. We have been back seeing one another and sleeping together. I love my best friend and feel so bad about keeping a secret from her. I just hate how she reacts to things and that's why I didn't tell her in the beginning but now its gotten worse. I wanted to be able to talk to her and I hate keeping things from her. What should I do? How can I get pass lying to her? I keep thinking that if my boyfriend and I break up again it won't be a big deal but we are doing good this time and besides I just hate all the lying. Everyone else knows about him except her and he doesn't know that he is a secret. They don't hang out because they don't get along so I'm not worried about them seeing on another.