I am no longer in love with my girlfriend, but I also do not want to hurt her.
What do you think is the cause of you no longer loving your girlfriend?
She knows I don't love her anymore but will still do anything to protect her, even if she cheated on me prior to this.
Being honest about your feelings is the best way to go about not holding on to ill feelings. The longer you hold on to these negative feelings the more they will continue to rise to the surface.
If I tell her then she's going to start crying, even if she's been bad, I know it's better to let her go.
Well the only way to fix this situation is to just break up with her. You are going to have to deal with her getting upset, but it will be worth you now being single. You are also helping her out since everyone deserves to be with someone who loves them.