WDID, that is wonderful! I think this will be a big step in you and your H's healing. As GF said, total transparency is important to build back trust.
It is so good to see you doing everything you can to save your marriage. I think the total transparency is a hard thing to grasp for the unfaithful partner. My H feels like when he doesn't tell me things he is protecting me and my feelings....when in reality the secrecy is what the problem is and the secrecy is likely how the R got as far as it did. When something is hidden it puts you back to step one.
I am just curioius, did he want to read the letter?