Long story follows:
We're both 35yo. A 10yo daughter and a 4yo son. Several problems after the boy was born, because I didn't accepted him very well (now I see I was both jealous and unconfident I could be a good father) and we suffered serious economical problems. I developed and told her about sexual fantasies of infidelity, and she accepted them. However the fantasies became more and more real, and then I became jealous and soon discovered she had a virtual sexual friend. We argued, and she promised to stop talking, but still contacted him every several months via cellular messages. Then she became "in love" with some coworker, but she promised to keep faithful and I believed her when she told me nothing really happened, this relationship has been fading. She said she felt I didn't care for her, and she felt alone. After increasingly worst fights, I asked if she had been unfaithful and she confessed a supposedly one-time sexual encounter with another coworker, who "supports her, cares about her, and protects her". She recently realized this 55yo married man physically looks like her father. As a side note, she has a sad infancy story, always alone, having to fight against poverty and a medicine career, poor support from her father, her mother died when she was 10yo (by the way, our problems increased when she reached her mothers age at dying, as she started to think she shoul do more "things" before dying), there are clues which make me suspect a sexual abuse in her infancy, and her loved stepmother died about one month ago (it seems that just before she cheated). To make the matters worst, it seems her stepmother somehow believed my wife was a victim of intrafamiliar violence (just as my stepmother was, by the way), and told her very bad things about me just before dying, including: "don't stay in a nonfuture relationship as I did". I can't understand how my wife is very resentful with me for this, even when she knows better than anyone I never were violent with her (in fact, sometimes I believed she was psychologically violent with me, telling me deceptive things and making me feel as a nonvaluable person). I always thought we (my stepmther and I) had a very good relationship!, this came as a total surprise.
My wife accepted MC (one session already) and IC. She was diagnosed a long time (about 30 years??) distimic disease complicated with major depression and started strong medical therapy (the phychiatrist thinks it will take 2-5 years of treatment)
I love her, don't want to lose her, but still I don't want her to cheat me (¿again?/¿still?). She says it was a one-time sexual encounter, and that she hasn't talked with this man since then (except a short telephone call some weeks ago). Still, they work in the same building, and she doesn't want to change jobs because she recently was offered a very good opportunity to grow. When we fight, sometimes she talks about going with him as an option.
I'm very confused. I really don't have any clue about this affair except for her words. Maybe it is over, maybe it isn't. She doesn't want to talk about this, and I'm waiting for the MC sessions for this. Of course I doubt of every man we know or works with her.
I don't know if I should hire a private investigator to assure the affair is indeed over. I don't know if I shouldn't press her too much because of her mental illness and recent therapy. I don't know how can I trust her, but I don't want to lose our marriage since I really love her

I'm a nerd, introvert and shy pediatrician. I love my children, don't mind about home labors (I cared my daughter from 0 to 3 yo while my wife studied in another city, and I was so happy), and work about 90-100 hours a week. I have very few friends and just one really good friend. I have been diagnosed with an obsessive-compulsive personality (but not an OC disorder), with some masochistic aspects, and I'm taking psychoanalysis therapy.
Just to share another person's point of view, my best friend tells me my wife considers me as unconditional, and has even told me to consider another relationship (she later apologized for this, she says she was very angry with my wife)