Hello...
I am sorry for your pain and situation, however, I suggest letting it die down. I am not an expert by any means, and sometimes I need to practice what I preach, but from what I am learning and what successful marreid couples tell me, is that you need to let go of the past. Technically, you two were not together when he was with this other woman, so this should not really be a concern, nor should you allow it to interfer with potentional of your marriage. Reading the letter was where it all went wrong. There hsould have been enough trust there for you to put the letter down and pack it away for storage. But you have to remember, who he was when he was with this other woman is not who he is now, nor is it the person who loves you now. People mature, their outlooks on life, love, and relationships change. We all change as individuals over time and I am sure, if he says he is done with her, he probably is. He has decided to marry you and share the rest of his life with you. You cannot change the past, and it should not change you and your current relationship. I completely understand why you read the letter and I understand how you feel, but you should know in your heart that he is your husband and he loves you and he wants you. After some time of him not answering her calls and not talking to her, she will go away. He should respect your wishes for him to cut off any communication with her as you should respect his when he says he is done with her. Don't let something like this get in the way of potentially beautiful future.
If you can... erase this from your memory and move forward with the love and trust in your heart as the day you said "I do".
I hope this helped and made sense...
Best of luck...