why did i do it to my first love???

It all started last year.. me and my present boyfriend split up, and i started a new relationship with my boyfrient childhood friend. wel that went wrong and me and my present boyfriend got back together. However.. recently my ex has came back and is causing my relationship at the moment to fall apart. I always said i wouldn't cheat.. but i have! its the biggest mistake of my life so far, iv not only made a mistake.. iv hurt alot of others too!