If you find it easier to hand the letter to your SO and have her read it while you are at her side, then I say that's fine. I think it might be best to rewrite it, however, as too much information as a precursor to talking with her might not be the best way to go. I think it's important that you be there to discuss it and try to answer her questions, and calm her down if necessary.
There is rarely a "good" time to tell a loved one about this. Your SO is dealing with her grieving at this time, so it might be best to hold off a bit, but just be aware that there will always be excuses not to talk to her about it -- bad day at work, issues with finances, health, the rotten neighbor next door, etc. It will come to the point where you just have to be determined to have a heart-to-heart conversation. Just be sure not to get too defensive or overly audible if her immediate reaction is less than you hope for. Keep calm, no matter what!
Good luck!
