You're an absolute moron. There's nothing to fight for. Your entire relationship was based around a lie that he wasn't seeing someone else. He wasn't faithful to her, why the hell do you think he's capable of being faithful to you? He put your health in jeopardy by sleeping with another person behind your back, and hers, and didn't care about either.
I don't get people like you, you're so stuck in your own little world where you feel you can fix another person's problems, "steal them back" when they leave you, and pretty much lead a guy around by his nose. I'm sorry, people don't work that way, and neither do successful relationships.
The real question here is why aren't you standing up for yourself, what he did to you, and working to ensure that you do not end up in another relationship like that.
What you're considering attempting will fail because it doesn't place the impetus upon him to change himself. It doesn't make him decide to change for the sake of his own sanity. Change for other people is temporary at best, but most often a sham and a lie to keep someone around that they truly do not deserve.
The right answer here is, "I am willing to forgive him IF he decides to stand on his own two feet, resolve his situation on his own like a real man, and when all of that is done, he no longer talks to her or her family outright, then we can talk about a future for us... If there still is one."
You're not placing any of the blame on him, and all of it on her. Which is pathetic. It's his life, he needs to fix it if it truly needs fixing.
What you're posting on here is a request for an excuse to behave poorly, and for others to justify your actions for you. We're not going to do it. If you already knew what you were going to do, then why did you come here to ask advice?
Either you're open to advice, or you're an idiot. You're most likely NOT the exception to the rule and in the end will just be hurting yourself.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."