Hello Colorblindexcuse, and welcome.
"Just" porn would be watching with out interactions. Photos, sites where you see already loaded material
and the like. This is different. This is connection. This is seeking out someone else, to have sexual moments with, together. This is reaching out for someone else, and giving them intimacy, time, and attention. In other words, betrayal, infidelity, cheating.
Heck, if it was so darned harmless, then no reason to hide it, eh?
You hide what you're ashamed of. You hide cheating. You hide betrayal.
Thing is, the only thing that you do have control of, is you. You can't control him, nor any other person. You, are the only thing that you can truly control, so please don't even go into the I'm Not Good Enough, If Only I FillInTheBlank, and so forth. His cheating is 100% on him. You didn't cause it, you can't make him stop it, and you can't control his choices to cheat. You can only control you.
Please start reading The Healing Library. You can find that in the upper left corner of your Forums screen, in the yellow box. Knowledge is power, so start reading. Good information written by experienced people. Next, take care of you! With your eating disorder, you are going to have to be extra vigilant about The Infidelity Diet, where you can't keep anything down and lose drastic amounts of weight. If you can't eat, and I threw up a lot just from stress, then keep a whole nutrition drink near you and keep sipping it. That will help with the dehydration too. If you had a good IC (Individual Counselor, the abbreviations that we use are in The Healing Library) that you used previously, please contact them and schedule an appointment. I also strongly suggest that you postphone that engagement and/or wedding date. The only thing that would be worse than this betrayal would be to be in the same place in a year, five years, fifteen years, with more property and children also tossed into the equation.
IMO, you both need counseling individually. You to figure out exactly what you need, deserve, and want in a safe partner. Him to figure out why he found it OK to make the decision to seek sex outside of your partnership. He's not a safe partner for you right now, and it frankly sounds like he hasn't been a safe partner for you for a while. This is major re-evaluation time. Shut down the words that he speaks, and look only at his actions. It's his actions, or lack thereof, that will tell you who and what he truly is.