He cheated and I can't forget

He cheated on me with his 'best friend.' He would call her up whenever we had problems and they would talk about me. I never was jealous or suspicious of her because he assured me she was 'like a sister' and she was my friend too. I had no idea, and he started getting caught in lies and she made some comments in emails to him that made me suspicious- I confronted him and he denied everything. If I hadn't felt that intuition I wouldn't know to this day. I constantly feel terrible- it haunts me and many times I have wanted to end my life. Please don't cheat, and if you do, have the decency to break it off with who you are cheating on. Be honest. No one is immune- he's an eagle scout from a small town and with a happy middle class family, never done drugs or drank or swore. We dated 2.5 years...and even though we are still together our relationship is hanging by threads cause of this.