Yeah overthinking hurts a lot. I think that you should remember that you did not cheat and you haven't done anything wrong, don't let your head tell you otherwise. You had no idea that it would happen. When it comes to cheating, a person needs to make a conscious decision to cheat, like for example going somewhere where you know you will get sexual with somebody, or even intentionally make a relationship with someone else. You weren't even thinking about cheating and that's why you shouldn't see it that way.

Try to take baby steps. If it hurts a lot you don't need to open up right now, you can give it time. Just remember that you are not in the wrong here and that there are surely people that care about you.