MissCat - you're getting some great advice already! I am so, so sorry you find yourself here, but you couldn't have landed in a better place for understanding, compassion, and advice that really works.
The fact H wouldn't look you in the eye? He was sneaking out the door on his bike without saying a word? That's guilt. He's done something. It will hurt. No question. But this is HIS mess. He's making all the mistakes, and the day will come that he'll have to clean it all up.
I know you sound really composed, but there's enough in your stories that I can tell this entire thing really upsets you. It will. The best thing is to really take the best care of yourself that you can. When people get stressed, they start to take shallow, short breaths without even realizing it. That causes all kinds of physical symptoms, as well as making you feel even worse than you already do.
Take some time, every time you think of it, and just breathe. Long, slow breaths - in and out. You really want to fill up those lungs, and slowly let the air back out. This will do more for your overall sense of well-being that I can describe.
Possible OW sounds like a hot mess. She is doing herself no favors. If people at work already attempt to avoid being around her, it's going to get a lot worse for her. No one thinks much of a home-wrecker. The less you say, the more dignified you are about the whole thing? She's going to look like a piece of crap in comparison. Don't worry too much about the party. If friends from work are there, and they see your H there with her, it's not going to reflect well on either of them.
There will be the drama-seekers who enjoy whispering about the entire sordid mess, but most of your co-workers are going to be solidly behind you, supporting you. She's going to find herself even more short of friends than she already is.
H? Not much you can do about him right now. The month before bomb drop, H and I were house shopping. 10 days before he told me he wanted a D, we put an offer in on a house! We were getting along great, holding hands, planning our future. Almost overnight, his attitude changed. Our marriage wasn't perfect, none of us wind up here if it is. I know, for certain now, that he was introduced to OW in that time frame.
To hear him tell it, I am the worst female on the face of the planet. I laid in bed for 21 years, and made him be a single parent, working full-time, raising five kids, cooking, cleaning, laundry, etc. He really believes himself!
None of it is true. He's completely rewritten our life together. Heck, I'd divorce me if that was the case! But it wasn't. I don't understand the mechanism behind the rewriting of history. I wish I did...it's puzzling.
The less you try and contact H, the crazier it will make him. He wants to think you're waiting at home, wringing your hands, and pining away for him. Even it that's true (at first, that's what most of us are doing) don't let him know that! You need to portray calm strength. You're fine without H. As a matter of fact, you're glad this happened. You needed to take some time for yourself.
This will do a couple of things. For you, it gives you some dignity to shield you, allows for a great mental attitude, and gets you moving forward on this long, long road. For H? It'll make him seriously think. You won't be acting the way he expects, and it will make him curious like nothing else could possibly do.
The more you get him to think, the better your chances of working this out. It sounds all kinds of wrong - but it works. Get busy, find people to hang out with. Investigate churches and see if there are any you enjoy - lots of social functions with those, too. I know you said you weren't religious, but you didn't say you were anti-religious.
Do girl's nights with your friends. The busier you stay, the more tolerable the situation will be for you. OW will be furious, too. You're supposed to be a crippled shell, instead you're out having a great time.
That's probably enough for now. You're really lucky. You got here early. Most of us careen around, destroying everything in our paths before we find our way here.
