If you want to rebuild and make the relationship work, don't do it.
Cheating on your wife won't make you feel any better, it will cloud your mind more. If you and your wife decided that she made a mistake and you both wish to move on from this situation together as a couple then definately do not do it.
You forgave her, you shouldn't still be holding against her, that's not fair.
You made your choice to stick with her, concerntrate your efforts on trying to make your marriage work.
If you are not sure that you want to be with your wife as you can't trust her, I would seperate.
I had to do this after I found out my wife cheated. We tried hard for a few months to get over it and make it work but I just couldn't forgive her. I thought about 'repaying the favour' but the hurt had been so full and intense that I couldn't go through with it, I felt too emotionally and mentally tired to create the situation again.
We seperated for a while so I could get my head straight and after a while I felt strong enough to turn away for good. Walking away was the hardest thing I ever did and I missed spending everyday with my son but in turn, I feel so much better outside of the marriage now.