Flinky: I had exactly the same issue with my ex. I knew something was wrong, she wasn't cheating on me, but when I couldn't get the truth from her own lips I read her diary... what I found cost me all the love I ever had for her.
People who lie just are not worth it, that's the bottom line. If they do it once, they may do it a hundred times, forgiving them just gives them licence to do it again. I'm so sorry, buddy.