My Ex married my best friend. At the time I had small children. Do you think this is right or wrong?

My best friend LeeAnn her husband Don and my now ex Richard used to have dinner together and hang out.
Richard and I divorced and a few months later Lee and Don started getting divorced because he was having an affair.
I felt sorry for her and I helped her get an attorney, let her stay at my house, invited her to family functions and took her to church.
I found out that she was leaving my house with my ex husband as he would come over to my house (I thought we were trying to work things out) and they were going out and fooling around. I felt like such an idiot when I found out about it.
I had small children at the time and she went from being moms friend to stepmother.
People say "well you were divorced" but, where I am from (the south) you just do not go there.
She and her husband were divorcing because she wanted children and now she is married to my ex who cannot have any more children.
Most men think nothing is wrong with this. Women tend to take my side.
It took me YEARS to get somewhat passed this. I will NEVER get over it. Am I crazy?
I ask her what she tells people about how she and Richard met and she said, I just say "it's a story" You think?
It really tramatized me.
Please give me you oppinion. Do I have a right to hate her? or him?