New to this!

[Rejoice]

I'll try to answer in order.

I am 36, and he is 41.
We met at work 14 years ago, he was never previously married but had a 3-year-old and a two-year-old. I have raised these children and consider them fully mine, and I believe that for the most part they feel the same. We have one biological child, 11.
We dated and then lived together for 7 years before marriage.

His affair began about 2 months ago, it was a slow build of an intense emotional relationship. Love is being tossed around although they both deny having physical relations. He and I were trying to help her with an abusive ex-boyfriend, who happened to be his ex-best friend, when they apparently began to feel more for one another.
I suspected something was wrong the whole time, and eventually just asked him for the truth. He admitted on his own to the emotional affair, lots of texts, several phone calls, and lots of visits to her house.

The admission in itself was somewhat of a miracle, as in the past he has denied things even when I had solid proof. Which seems to say to me, that he is feeling somewhat guilty about this.

I have seen my doctor about the panic attacks and sleeplessness, and have a prescription. The drugs affect me somewhat badly, so I try to use them few and far between.

By an overnight change I mean he went from his usual mix of tenderness and fun and crankiness, to Stone Cold within the span of a day or two from telling me the truth.

Yes, my brother-in-law, my husband's brother, walked out on his wife and daughter a few months ago. This is definitely a reflection of their upbringing, they had two cheating parents who had an incredibly traumatic divorce situation in which my MIL abandoned my husband at 8 years old to move to another state, leaving him with his alcoholic and gambling addicted father. She took with her the brother that recently walked out on his wife, and he suffered a lot of abuse from her subsequent partners. After that. It was a really bad situation. My husband was often alone, the electricity and water would get shut off due to his dad's gambling, and he would have to find meals where he could.

I looked in his truck, and found the letter.
I was snooping, and I have apologized to him for that. I was quite concerned that since he and I are still sexually active, he may not be forthcoming about his activity with her and I may become subject to some disease. Not that that's an excuse for snooping.

Yes, he's previously had an affair, over a decade ago. I was able to mostly overcome it as he was repentant and it was clear it was just a physical thing--it hurts much worse that he's emotionally involved with someone now, as weird as that is. And it hurts far worse than that, that he's determined to split up our family and hurt our children over this.

I was 14 when my mother, sisters and I ran away from my abusive father. We all have good relationships now but my life was a wreck for at least a decade over it.

I think I covered his childhood trauma above.

Thank you for your insight.