The problem with this armbow, is that your default position is to trust someone you love this much. Your default position is also not to snoop, because perhaps it's something you've never done before and it feels wrong to you, being a decent person. I understand all of that very well indeed. What many of us have learned through bitter experience is that this default position was abused by the person we trusted most - many of us here too would have described ourselves as hugely perceptive - even with people we didn't know well.
I often wonder why, when I had suspicions, I didn't try to verify my gut instinct. It really isn't like me to avoid a confrontation, but my H was so convincing in his denials and had such a plausible explanation for his behaviour, that I was able to put doubts about fidelity out of my mind. This, allied to the trust default, meant that I was unsighted for a further 6 weeks or so. My accidental discovery was therefore an enormous shock and our recovery was greatly hampered by his earlier denial.
You say you are frightened of what you will find, but for me now, my biggest fear would be making decisions without all the information to hand. You cannot forgive until you know all there is to forgive - any recovery will be built on sand. Perhaps you feel this will be easier to recover from if no actual infidelity has taken place, but if he is lying about that, any future relationship is never going to reach its potential if one of you is holding a secret.
It must seem even more difficult to you because on the face of it, he appears to be doing the honourable thing, coming to you (a year too late, in my view) with his concerns about his attraction to someone else. It puzzles me greatly however that he is so troubled by what amounts to a crush, that he wants to leave and sort his head out, instead of working things out with you as a couple. This is what's making me sceptical that you are being told the whole truth.
You might remember another thread around Xmas time, when an H seemed to have disproportionate remorse for having had a drink with a female colleague, making over-the-top protestations to that colleague that "This Can Never Happen Again". Several of us queried a reaction that seemed disproportionate to the incident - and of course it turned out that contrary to what he was saying, something did indeed happen with that colleague.
What's your bottom line armbow? If it turns out he was unfaithful - and regrets it - would you be willing to attempt forgiveness? If you'd rather not know, but want to save the marriage, then accept what he says and try to build a new marriage.
If you really do want to know though, in your shoes, I would be trying to verify what I was being told, but I would also say to him that you are willing to work with him on saving your marriage, but can only do so from the position of total honesty. If it transpires later that infidelity did take place, that marriage will be rocked again and further damaged by his earlier denials.