I need help with this one please?

My marriage is going downhill. It all began when my husband took sides with his sister who lived with us when we got married and the sister and I could barely get along. This took a big toll on our marriage. He does not think he did anything wrong and even went ahead to have an affair. His reason: because I was fighting with his sister and he needed solace. As if the sister was homeless without him!! I have forgiven him, but the betrayal is so much I feel like going away to a place where he will never find me again! How can I get out of the bitterness trap. We are not talking and generally we are doing badly. We've been married for less than 3 years!!