Where to go from here..

My husband was unfaithful some time ago, but we have worked through it and improved in leaps and bounds. My problem is that I've since lost every friend that I had. I had a tight circle of friends. Our kids have grown up together for years and our time was always spent together. The person he was unfaithful with was a new addition to that group of friends and I was really upset that something like that would happen, especially so close to home. Since everything came out in the open I have obviously had a lot to deal with and instead of having my friends to rely on, they all left me. And when I discovered the affair my husband was out of the country for months. So I feel really abandoned. Now all they do is hang out with her. All of them. They have get togethers and do stuff with their kids together and go out together. All the things that we have done for years and years. And from time to time I get an invite to bring my kids to one of their kids birthday parties where she is also attending and it's gut wrenching. I can't attend because I can't be in the same room as this woman, but I have to explain to my kids that they just can't go. I just don't know where to go from here. Why are they all buddy buddy with her?? Why did I lose all my friends when I didn't do anything wrong? ?