Im so sorry to hear all that you are going through. My husband and I were separated for about a year when he was actively using, and he had an affair with a woman who used drugs and it went on for several months. I was pregnant at the time, so I understand somewhat your pain. Lots of lots of different emotions during that time for me. He ended that relationship, and months later went into rehab for his addiction. I had no idea in the beginning if we would be able to save our marriage after all that happened. But we were both committed to trying, and he was also committed to being clean.
One thing that helped me was private therapy; I was able to talk about all of my emotions, my anger, my sadness in a safe environment with someone who was trained to lead me through the process. Sometimes there is a bit of actual grieving involved, because there is a realization that what we had at one point was changed, the innocence or the belief nothing could ever come between us as a couple. But the good news is that sometimes a new relationship can be formed; with an even stronger foundation, and a deeper level of appreciation and respect.
Marriage counseling also helped us a lot. It helped us communicate, and walk through the pain of all that happened during his active addiction. And I had my own share of mistakes I made too; sharing these with him allowed me to heal. If your husband is open to this I would recommend it.
Journaling also helped me. Write out what hurts, and then define why it hurts. And then you can pinpoint what needs work. Maybe it is feeling insecure, or feeling afraid. Plus it helps to just get it all out.
My husband has been clean now for over 15 months. Our marriage is doing well. We have both learned a lot, and Im sure that will continue. I look back on the relationship he had and no, I don't think it was genuine. He was on drugs, and he was in a bad place emotionally. I think if you look at a rehab, you will see that it is also a very vulnerable place for people. Coming off drugs causes a lot of confusion, pain, and overwhelming feelings. As horrible as it sounds they were probably attracted to each other in that environment and thought they had some bond through their experience and the pain they felt. I don't think it even slightly compares to a relationship built over time, with stable emotions, and shared goals and dreams for the future. But I know it still hurts and betrayal cuts deep regardless of the cause.
Im glad you posted here, its a great place to express your feelings. Please don't ever feel embarrassed for what you have been through. Real life is messy.