I have been reading through this thread and I have great sympathy for you regarding all your girlfriend has done, and how hurt you are by her actions.
One thing that really concerns me is you using the word "Revenge" and you not knowing if it will be a sweet revenge or not. Dear TFiend, please don't go down that road, you may do or say something that you will really regret in the future. You may think at the time you are hurting your girlfriend, but in the end you will be hurting yourself as well.
All through your other posts, you sound like a guy who loves and cares for others deeply, that you have a strong work ethic, you help people out, have integrity, are loyal and trust worthy, you are honest and honourable in your actions. If you seek revenge, how will you fill about yourself later on?
Yes, your girlfriend has behaved despicably, you have every right to be hurt, miserable, sad and horrified by her actions. It may take you a while to get over the emotional abuse you have suffered.
I do not know this girl's problems, maybe she has mental health issues, maybe she is a person who doesn't see much wrong with have an affair with someone outside of a relationship, I don't know. Either way, you can be better and stronger than her. You can go and seek help, talk to a professional person about how you are feeling, continue to write here, what ever you need, but please, please, please, do not become all bitter and twisted up by what has happened. It will ruin your life and this girl may just walk away and continue living the same way she is doing now.
I came home from work early one day and found my husband in bed with another lady. I discovered she hadn't been the only one. Was I angry, did I want to take revenge? Of course. Did I become bitter and twisted about it all? No. I walked away and started a new life. Do the memories still hurt? Yes, but they do not control who I am today and how I now live my life.
There are many ways you can show her and yourself that you are a strong person, revenge is not one of them.
Please stop and consider the consequences of any actions you want to make in anger and while in pain.
Thinking of you, and sending your encouragement for a stronger you,from Dools.