Your lack of bearings is palpable. Considering you thought everything was perfect, and would not have had much of a clue had she not engaged in some questionable conduct, a few points are in order.
I doubt that simply diagnosing her as broken or lacking in character is going to get it for you. I think you are looking for more than that.
You are dumbfounded by the brazenness and by the lack of physical attractiveness of her boy toy. You can't see why she would risk everything and then when caught try to keep you on a string through bald faced lying.
So after a long time here and a lot of reading I think that a lot of the affairs we see here fall into a category that might be called detachment from reality. sometimes its called magical thinking, although actual thinking is not a real part of the dynamic.
When an affair goes on for as long as this one has, they are past the joyless fuck buddy acting out, past the thrill seeking, past the "I do it cause I want to and can get away with it" stage.
Usually, and I think here, they are at the limerance and addiction stage. It's no longer about sneaking around for a quick fuck. It's now all about emotions and brain chemicals taking over, all saying that all is right with the world. They see the AP at first as a perfect fit, a person who they need in their life, and overlooking the flaws is the norm. Is the AP married? No worries. They will find a way. Are they married? They will find a way cause they need it and if the BH/BW could only know how it feels, they would understand. They have found their soulmate. It's fate, kismet, karma. And they deserve this. If other people find out, they will be happy for them.
Hence, the look of joy when she opened the door to a very average cheater. There is no talking them out of it. They are addicted. That is why they will risk everything. This is why over and over here we see people who cannot understand why the spouse can possible be attracted to the ugly loser, still living with their parents, low payed jag. The stripper, the hood, the outright criminal. How can this be?
It's because people can convince themselves of anything when the brain chemicals that make them feel good take over. There is no right. There is no conscience, there is no self reflection, and there can be no serious consequences in their minds.
When Dday happens, the addiction does not immediately go away. Like all addicts they have to wean off the good vibes in their heads. But consequences start to drop, and they had no plan - none at all about how to deal with it. Hence, the crying and screaming and ranting and lying and promising and acting like they just got sentenced to the guillotine kicks in. Because everything is now in the open and insane emotion takes over. And that emotional reaction is no more productive than the magical thinking stage.
This is why it often takes years for a WW or WH to get their heads on straight, if ever. And the BH or BW has to have a great deal of fortitude and will to deal with it. Usually that happens when there is a reason to put up with it. Here, there are no reasons.
So this is a long way of trying to explain the "she is broken" comments. Maybe you knew all that already. But in reality, and in a very real sense, this has nothing to do with you, your size, your personality, your looks or your character. Do not take an iota of blame. From her or yourself. And is something that you cannot fix by being nice, or waiting around, or taking it on the chin.
I wish you all the strength you need to get through this. You will get through this. Know that above all things.