Hello, Fergus. Welcome to the greatest club that no one ever wanted to join. I'm sorry you've had to find us here.
For most people, the betrayal of infidelity is a profound shock and a severe emotional and psychological trauma. It took me several months just to get over the shock and several years to heal.
Your #1 priority right now is to recover from the shock. That means taking care of yourself. I could barely function in the first few months after d-day. I lost over 30lbs in about three months. I couldn't sleep, felt anxious 24/7, couldn't focus on anything, would walk from one room to another with purpose and forget why I'd done it in the few moments it took to get there.
It's a powerful shock, young man.
Try to eat small, healthy meals when you can. No McD's or other crappy fast foods or microwave snacks. Good meats, cheeses, fruits and veggies.
Stay hydrated! Drink lots of water.
Absolutely no alcohol or recreational drugs. They may dull the pain in the short term while prolonging the agony.
"Hit the gym." It may seem like a cliche, but the endorphins and hormones will help your recovery. Take walks or ride your bike. Get out in nature if you can, even in the depths of winter mother nature has her own healing powers.
If you can't sleep, try a mild OTC sleep aid. Follow the instructions carefully.
If you have a PCP, make an appointment and let him know what's going on with you. Be open and honest. If the doc is older, chances are he's heard it all before and will give you some solid advice about how to recover from the shock.
Therapy can help. You may have to go through a few therapists before you click with someone. Remember, you're hiring the therapists, so interview him or her. Ask questions about their knowledge, training and personal experience with infidelity.
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As others have advised, call off the wedding. Marrying this woman less than a year out from d-day would be an extremely poor and precarious choice.
In my own very humble opinion, I would advise you to ghost this woman. You are both still very young. Even older, mature old farts like me find examining behaviors and making changes very difficult. At barely 26 years old, I doubt she has the maturity or capacity to own and fix her shit, a paramount requirement for reconciliation.
As for the house, make arrangements through a neutral third-party if you can. Either one of you buys out the other or you split up the sale.
You WILL get through this. You will recover and heal. It takes time and a little effort on your part.
Know and understand that her decision to have this affair has absolutely nothing at all to do with you. Nothing you ever did or didn't do, nothing you ever said or didn't say, would have made any difference at all. Nothing about you, not your looks or physique, your successes or failures, had anything at all to do with her choice to have an affair.
Focus on you, your recovery and healing.