Well, that does shed a new light on the subject. You will have figure out what you can do in the way of forgiveness -- of which I believe there are two parts:
- letting go of any ill will,
- restoration of trust.
The first part, the letting go of ill will, is actually advisable just for the well-being of the forgiver. Even if the forgiven party does nothing to "deserve" the forgiveness.
However, restoration of trust is a more complex part of forgiveness, and requires a process of repentence on the part of the offending party. The party who was hurt should outline some kind of reasonable course of action the offending party can follow to regain trust. Then, the offending party must follow that course of action. Generally it will consist of three parts:
- providing evidence that the offensive action won't recur.
The girlfriend in this case was both cheating with your husband, and hoping/intending to shatter your marriage. So she has a considerable debt to pay before regaining your trust. As for her expectations and how she was hurt in the situation, I think that is something for your husband to make up to her. It's not your debt. Your husband should also take steps to regain your trust.
The good news is that, I think at least, relationships (and poly relationships in particular) tend to be harder in the beginning. The reason why it's good news is that perhaps it means you can look forward to things getting easier in the future.
It sounds like you and girlfriend could still use some time together, but only on the condition that husband will not be brought up in the conversation. You can decide later if hubby can become an okay topic, but for now I would try to have a girl's night out where you don't have to think about him.
Hope this helps,