People deal with trauma in lots of different ways and mental health issues mean people may want to withhold their emotional vulnerability for fear of where it might go.
People who suffer from depression have a tendency to attribute negative events to something internal, within them, and to attribute positive events as being caused by something outside of them, luck, or circumstances. An affair is a dangerous threat to an already vulnerable depressive state because the cheatee often takes the blame on themselves first anyway (especially women) and this could exacerbate to a spiral into depression.
People who have been unfaithful themselves are also in a danger zone when they discover a partner's affair because they deep down feel their own guilt and that their anger or distress is hypocritical. This again makes them look for internal blame instead of locating righteous blame outwards as is healthy, and repress emotion because it seems inappropriate or unjust.
This situation is a cocktail of emotional repression and self-blame, both very strong precursors to depression and I would humbly suggest that Orm consider counselling, where communication could eventually move on from letter writing to face to face, gently and slowly with a safe 3rd person. I also sense a gentle but steady sense that you feel wither guilt or unworthy is relation to your partner, Orm, and that is something separate from the affair that could really do with being addressed as an unequal relationship is a tinderbox for infidelity on both sides (as one person may search for self-worth they lack elsewhere, and the other for the partner they 'deserve'.)
Orm has also found someone she considered part of her support system to be not just absent, but possibly even contributing to her depression (it's obviously a cyclical thing). This is devastating and it's really important she has somewhere to come that's strong and listening that ISN'T her OH, like here.
Read what he writes with an open heart and a sceptical eye, Orm, if you can. he will almost certainly try and track the affair alongside your relationship, as in 'around the time you were unwell or withdrawn, I found myself getting closer to OW...' This sounds completely sensical and is comforting in a strange way because it places your relationship as the driver, but people often try and make these things make sense in retrospect. It's just as likely that he would have done it anyway, that it is about him and his stage in life. And never forget that a supportive, faithful OH would have been more supportive and mor epresent through times of difficulty, not less.