I don't necessarily think it is karma, you were seeking happiness and you got involved with someone who hurt you. True, it is not right to find a new person while you're still in a relationship, but person #1 wasn't making you happy. The second person didn't make you happy either. Instead of focusing on them my best advice for you would be to focus on yourself and how you're going to be happy as a single person. You may meet someone yet to get married and have children with but hopefully it will not be an A.
I also became very angry at myself for getting into the relationship with an A. Honestly I did not know how much it would hurt until I was far too deep into it. I am still hurting a lot too, but it is best not to blame yourself too much, i think.