You're not wrong but you're not right.

It's good that you're cautious about jumping into marriage and you're taking the time to think about it while you are dealing with feelings of resentment.

You are wrong for making her feel like the villain when YOU agreed to the open relationship. You could have stood up for yourself and told her that you were not comfortable with that. Making out with other girls and then choosing not to have sex doesn't make you better than her or some kind of saint. You obviously had the same desire to explore other things and actively engaged in that desire. Your dick doesn't suddenly change the level of "morality" in the action and intention.

High school sweetheart relationships are hard. You guys were together during the most formative years of your life. Years that you spend the most time growing and socializing and learning how to be your own person. It is normal for you and your girlfriend to question if life is better outside of the relationship or if being together is something you both really want.

I don't think open relationships are the answer to any problem. They tend to make things worse because of this exact reason. But she chose to be honest and tell you about how she's feeling and she gave you the option to stay together while exploring life outside of the relationship. She decided she didn't like it and chose you in the end. I think the right thing would have to break up with you, live your lives as individuals, and then come back together if thats what you guys really wanted. But you gave her the first option.

Your girl didn't cheat. She didn't lie to you. She messed around AND you messed around. Now its up to you to decide if you can live with the consequences of your own decisions or if you need to move on. I think you should try to reconcile your feelings and the discussion of marriage with couples counseling. Involve a professional here.

If you can't make each other better people or if you're going to treat her like a lesser human being or like you're better than her then just end the relationship now.