I'm sorry you are here.
Mental movies are very difficult. I tortured myself with them for months.
Your last twelve years together hasn't been a lie. I understand why you feel that way. You might doubt lots of things, question things...wonder if it's happened before.
I agree with much of what Razor wrote. I have to add, ask your husband for all the details. All. The. Details. For me, that stopped the mental movies (my hubby went to a massage parlor). Ask for the details over and over again - that way you can be certain if he's telling the truth or minimizing things like Razor suggested. Liars can't keep lies straight. Let's face it- anytime anyone gets caught doing wrong (not just cheating) they minimize the facts. Just calmly tell him you need to know all the details. And the next day, ask again for the details, and so on and so on and such.
It's great he's giving you full disclosure on his cell, etc. Tell hubby he lost his right to privacy- you see his cell phone every day, you see the phone bills, you see the credit card bills, you see all his online emails/clubs etc whatever, you get to count the money in his wallet when he leaves for work in the morning, and you get to count the money in his wallet when he returns and he needs to show you receipts when he goes to bars or restaurants, perhaps tell him no more bars without you, you get to track his location via his phone-if that's not an option, well he's sending you selfies every hour or every half hour or every ten flipping minutes. He needs to earn that right back. And you don't have to be _itchy about it, just say "let's see it." And if he has to travel...then firmly inform him - you need selfies constantly.
Lastly, go get tested...even if he says he wore a condom....I'm sorry to write this, but if there was oral sex or kissing...you could be at risk. There's a lot of scary, drug resistant STD's out there...including ones found in the throat. I'm worried because this happened months ago and you are learning it now. My situation was similar. I was so angry my hubby did what he did, but even more angry that he didn't tell me right away. He did do STD testing, but didn't do it correctly - you must do it over a period of 6mos to a year and during that time, you must practice safe sex.
I would suggest marriage counseling. Ask your doctor or your pastor who to recommend. We picked someone off our insurance web site and the counselor was a disaster. Spent six months attacking me.
I'm 8 months out and healing.
Wish you luck.