Forgiveness is a splendid thing, but vengeance is much sweeter.
Betrayed and abused by medieval writers, Guinevere was never disloyal to Arthur. It was all a lie made up by her enemies who wanted to bring her low and humiliate her. Unfortunately even her friends and allies were all too ready to believe the lies instead of the truth.
To the day she died, she was loyal to her husband and while she and Lancelot were good friends, that was all they ever were.