I have only just seen this thread.
I have recently been through something similar with dh (he's the one who said he didn't love me, he sent an inappropriate message to another woman even though nothing had actually happened between them) and so I think I have a really good idea how your dh feels right now.
I think if you could write down your feelings about your marriage and how you feel about dh that would be a good start.
It seems to me that even though you're understandably devastated over the letter you need to ask yourself... is it the thought that things ending 'this' way that has upset you, but once this is sorted will you end it anyway, or has it made you realise that you actually want to try and work things out and give your relationship another go?
I'm waffling here - what I'm trying to say is - when you speak to him, you need to try and be clear what you want. Telling him how sorry you are etc but then turning round and saying 'now you believe I didn't cheat on you, I still want a divorce anyway', IMO would be cruel. You have to try and be really honest with yourself and him from now on. If the letter was a reflection of your unhappiness and your decision to end the relationship, then he needs to understand that from the start.
For what it's worth I don't think you wanted him to see the letter or left it there on purpose, I think you're extremely confused. Maybe writing down how you feel about dh and what you think you want from your relationship and for your future will be the best way for you to move forward
xxx