One of the things I have noticed about people is that they tend to expect everyone else to act the same way they believe the would. If your wife never trusted you, it was because she always suspected that if something came along, she would cheat. You trusted her completely because you you could not imagine yourself cheating. Just one of the little warning signs I picked up on through life, a little to late to be of any good for myself.
Right now you are still in the shock phase of discovery, and the info that you dig up will reveal other things that as you piece them together will shock you some more for a while, some will also make you very angry, but the more information you receive, the better you will be able to understand what happened, and make the decision about which direction you should go.
If you can find out who his fiance is, she also deserves to know about this, and there is a very good chance that the other guy will back off if she knows. Not guaranteed, but the general trend when men are discovered is to throw their AP under the bus and focus on their primary relationship. He will also seem less attractive to your WW if he is scrambling and in a panic to fix things, has to start hiding stuff from his fiance, has limited time to communicate with her, and it always has to go through something secret to keep it from his fiance etc.
It is your turn to decide what you want now, but you don't have to, and probably shouldn't make any irrevocable decisions until after you get through the worst of the stages of grief and can think clearly.
If you are like most people, you are about to learn about what we call the infidelity diet. It is great for rapid weight loss, but murder on your health.
Drink plenty of non alcoholic liquids, and throw in a few sports drinks to keep electrolyte balance, you need those not just for physical reasons, but also to help keep your brain functioning. I liked the blue flavour of gatorade and powerade myself.
The healthiest food for you to buy now is your absolute favorite junk food. Preferably something small and bite sized that you can just grab one of whenever you feel up to eating. You may feel like you are stuffing yourself while still rapidly losing weight. It is not a permanent thing, so in the meantime accept the one small pleasure of eating crap whenever you want it. Potato chips have lots of vitamins and minerals, and the salt will help with electrolyte balance. Chocolate has many wonderful properties, and was issued to soldiers as a health food with a high energy content to allow them to function in extreme environments, and contains a natural anti depressant, dark chocolate is even better. Hard candies are recommended for winter survival kits because you can slowly suck on them and they are almost pure calories. Donuts are just a piece of bread loaded with sugar and fat, almost an ideal food for short term survival. When you are feeling a little better, you might want to consider protein shakes, either the sporting variety, or the meal replacement type for dieting.
Don't hesitate to see a doctor for some anxiety medication (plan on short term use only), or possibly anti depressants. Don't feel ashamed about doing this, it is normal situational depression and anxiety, not a chronic mental health issue. You are supposed to be depressed and anxious over this.
Try to avoid caffeine, you will have enough trouble sleeping already. The exception is if you need it to function for work after getting minimal sleep the night before. Sleep whenever you can, even if it means letting some household stuff slide. This is also normal.
Most of all, know that you are not alone in this. Right now, there are an estimated 8 million Americans going through the same thing. You found us, and we will help you through this. We will answer questions, explain behaviours, help you understand, and help you decide how to proceed based on the information you have, what you want for an outcome, and the knowledge of over 36,000 people who have been through this, and collectively read almost every book, study, serious article, and recovery program in existence.
It will be very tough for a while, and recovery is generally long and not easy, but we will see you through it.