Most Helpful Guy
There is a 12 year gap between you and him. That pretty much means that he doesn't really consider age when he is looking at people. He will look at how they make him feel, how much he likes being with them, what he thinks of their personality. So for him, there really won't be much difference between a 12 year age gap and a 25 year age gap. Not at his age. It will simply be "She is a lot younger".
It's shitty for you, but he won't be thinking about it like 'she is young enough to be my daughter'. He won't care. It will simply be about her as a person. The thing to keep in mind is that if he left you, the relationship wasn't right for him. It doesn't matter that he found someone else, or what their particular qualities are, it was that he wasn't satisfied, and was therefore looking for something different. The relationship wouldn't have worked anyway. If it wasn't her, it would have been someone else.
I'm really sorry to hear that he was leading you on and making you think you had a good relationship while working on something behind your back. That's a shitty thing to have happen, and reflects poorly on him as a person. But not focus on the age thing, that won't be the root cause.
Most Helpful Girl
It's definitely not normal, but unfortunately it's not unheard of. Many older men are constantly seeking younger women. Why? I'm not sure. There's a lot of theories. Some people say it's just because the younger women are prettier. Others say it's almost perverted (technically she's a legal adult, but there are many who would say she's still basically a child, and this is the closest he can get to f*cking a "child" without going to jail).
I'm sorry this happened to you. But it sounds like you had a narrow escape! I'm glad you found out before you married him. Even if his new relationship with her doesn't last, I don't think you should wait around. He's not good enough for you.