MM(34) in a DB. Have a for sure AP. Never dabbled in it.

So a little back story. Been with one person(highschool sweetheart) other then my wife. Wife(32) and I(34) have been married 13 years. Crazy sexual compatibility up until her auto immune diseases started. So, my DB isn't the same as a lot of others. It's a DB regardless, we've had the talks a few times. Between life and the diseases, general intimacy just isn't anywhere near the top of her priority list. Unfortunately for me, that's my love language.

Now to the subject of this post. I have a 100% full proof MW AP who is completely on board and is basically waiting on me to make the first move. As I said in the title, I've never dabbled in infidelity. Grew up in fairly religious southern Baptist home. However, I no longer follow any religion, I'm a straight up athiest. How does one go about getting into it. My main concerns are, what if I have zero remorse and it opens up the flood gates or what if I'm completely guilt ridden. What emotions am I to expect? Is it the same for everyone? How does a smooth transition to a full blown AP/FWB occur?

Edit: just for some clarification. My wife and I have a "happy" marriage. I provide her with everything she needs. Monetarily, emotionally....objectively she's taken care of in every sense. I think I'm a good husband and father, and I highly doubt she would have any objection to that statement.