From all you share I can clearly see this is not what you want, and that you are possibly hoping we can wave some magic wand to help you make it all better...
I’m so sorry. We can’t...
The only way to deal with this type of issue is through reality, and through taking what he’s saying – both directly and indirectly – seriously. The first part of this post deals with the reality, and then I might share what is more in line with the hope you wish for.
Let me start though with the mantra we use here: Their decision to cheat is NEVER due to us. We can be terrible at marriage, but that allows them to demand change, not cheat. So don’t take this as some failure on your behalf. If anyone is flunking spouse 101 it’s him.
It’s a bit like waking up to the smell of smoke in your house to the shrieking of the smoke-detectors. No matter how much you wish there was no fire then chances are there is, and that no matter of reorganizing or moving furniture will change that. You can only decide how to move on once the fire – the crisis – has been dealt with. Only then do you know if the house/marriage is salvageable or not.
My first suggestion would be to ask your attorney if infidelity factors in any way or form IF this goes to divorce, and if so then what sort of proof is required. Chances are it doesn’t but be 100% clear on this. If it does – well... then get the proof required. If not, then skip it.
Be very clear on your rights and have a clear vision of your requirements if this ends in divorce. Be realistic. You won’t get everything. Divorce is this strange mathematical enigma where somehow getting half of what you have seems so much less than having half of what you have now. If you can, then envision your life 2-5 years from now, based on what your attorney suggests is a realistic outcome from a divorce, and then aim at making that the best version of your vision.
Be realistic... Amicable is great, I would never suggest making a divorce harder than it needs to be. But D is a conflict and "amicable" is a bit like the surgeons wearing pastel instead of green scrubs. The goal is that once divorced, you two are great coparents. It’s not that you two are friends and visit for coffee and all that. You wont be interested in his dating life and living accommodations. Envision this scenario: When your 19 year old get’s married at 25, you want him NOT to have to worry that mom and her new friend can’t be in the same room as his dad and his new wife. That is basically the only goal – if this ends in divorce then he becomes a fond memory that no longer impacts your daily life.
The hope part.
Is he depressed? Has he ever been diagnosed as depressed? Any medications?
Some of what you share made me think that might be the case. If so – can you get him to reevaluate his treatment?
What you can do is ask him if he doesn’t think that maybe the years you have and his sons deserve that he gives the marriage a chance. That if he commits to MC and guidance until the younger is 18. Of course, this will require that he ends it with the 36 (and btw- is she married?).
Ask him how he sees divorce. Does he have a suggestion on the next steps.
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Other than that – Listen to leafields. Take care of yourself physically and mentally. Eat. Exercise. Hydrate.
Find a good friend to share the situation with.
Lean on people – these situations are the ones that show you who your true friends are.