This guy is in a bad marriage and latched onto you. He probably sensed you were bored and he moved in on that. You of course are complicit in this as you are an adult.
Time to step back and reconsider the 'best guy anyone could ask for'.
Consider him instead of this guy who had and has A LOT less to lose than you do.
You need counseling. Talk to an impartial observer about your marriage. Get to the root of why this happened in the first place. Think about your next steps with a clear head and not in a hormone-induced haze.
Stay or leave as you feel is best, but do talk to someone. In particular, make at least some of the topic about your children, and about your economic situation. Spoiler alert
: a 13-year-old is usually hit rather hard by divorce. Second spoiler
: your financial status will likely suffer.
I am not saying you absolutely must
stay together, but those are realities that should be in the mix.
Notice how I am not suggesting you base your stay/leave decision on this guy? That's because such decisions need to be made by you and you alone, or at least in conjunction with your husband. And maybe marriage counseling is also a good idea. But leave the affair partner out of it. He has clearly moved on.