It all hurts. There is certainly no way around that. The immense pain is just a horrible reality of the shit storm that they created.
I don't know what it feels like to have to learn about multiple affairs. I don't know if my ex was with anyone before he got together with the slunt, but I'm sure that would hurt me even more.
So, I'm like you - my ex cheated with the slunt and they are still together. I can tell you this - the slunt is not special and neither is the pig that infiltrated your marriage.
In the beginning, I felt exactly like you. I felt like she must be awesome for him to have left us. I truly believed that she was a younger, more fun version of me and that he got tired of the responsible mom that I became. He said he wanted to live alone, travel around, follow bands and essentially act like a teenager. She supported that and supported his drinking habit. She was a total package.
So I thought.
Unlike you, I have time and perspective on my side now and I have seen that the slunt is nothing more than a stupid, possessive, controlling, nasty jackass who has zero respect and/or love for my ex. This wasn't a true love story. He got caught and she was the easy choice. She sold him a bill of goods and he sold her the same. She made it seem like the grass would be so much greener over on her side - no responsibility, free wheeling, part-time parents, and fun all the time. Guess what? He now owns a house, does all the grocery shopping, takes care of the lawn, cleans, drives her kids to school, and essentially has about 1,000 times more responsibility than when he was here. On her end, she was looking for a well-to-do professional who could be her hero. He's far from that. He's physically lazy, emotionally stunted, and financially strapped with school loans, child support, a mortgage, and significant credit card debt.
They both got exactly what they deserved. I sometimes get sad that they are still "together", but when I hear from my kids that he sleeps on the couch or with my kids at times; that they fight when the kids go to bed; and that she screams at her kids all the time and does nothing to discipline them when they are mean to my kids, I see very clearly that she's nothing special. In fact, she can't hold a candle to me and, no matter how hard she tries, she never will.
There is a thread in General that has been going for years now - it's called "Honey, they always affair down". If I knew how to bump it for you, I would. Nothing could be closer to the truth. I promise you - if he stays with her, you will eventually see the "real" her. They can only hide and show their false selves for so long.
I know it's easier said than believed, and I know how much you are hurting right now, but your worth is not tied up in him. He's a busted, broken down dude who gave up a family for someone who was willing to lower herself to be with a married man. There is nothing special about either of them. There will be a day - I know you don't believe me, but it's true - that you really see that and take it into your heart. That will be a good day for you.
Until then, take care of yourself, try to heal, surround yourself with friends/family and SI. It won't feel like this forever.