I sincerely understand. There are so many details to remember and human memory is so unreliable, particularly under stress, that you simply don't recall everything.
The best I can say is start writing things down. Not for your wife, but for you. Fill in the gaps and check your correspondence. It's going to be hard, because you are going to want to avoid putting things down fully or accurately. It's going to hurt like hell. You're going to avoid and weasel the truth, but you are doing this for -yourself- and you already know the truth, so write it down.
Don't show her the contents, this is your safety net to try and be honest with yourself. After you have worked on it, use these as notes for yourself so that you can tell her about it. Any time you shy away from the truth, or get scared and avoidant it will be there on the page in front of you. Think of it is your reference materials for a presentation.
I didn't have it printed, or even complete, but this is how I got through my full confession yesterday. I knew the hard truths were typed up on my computer, waiting for me to come back to them. I had faced the truth already, which made facing my wife possible.
Who knows whether my marriage will survive at this point. I have to say, that at least it feels like a clean wound now rather than a festering one.
I wish you courage in this. I was there yesterday. I know how you feel.