To everyone who has posted: thank you I really appreciate your comments but I think it is time for me to bow out, mainly because I have work tomorrow and need to be a massive loser and sleep 
but also because this has rather taken on a life of its own and many of the things being brought up have not actually been things I've said (e.g. They've been married for twenty years, that he's Italian, that he slags her off). I also, don't really want to read any more comments that involve 'you are stupid/dim/moronic', luckily, I'm secure in my intellectual capabilities! But before I go, I just want to say:
Every relationship is different and I firmly believe that you can't apply blanket rules to them, just because some men make up email addresses etc, it doesn't mean that all do. I also think that you can't choose who you love and when you meet someone and they make you feel safe, secure and happy then that's great, no matter whether you feel you have a future with them or not. In my situation, I know we won't have a Disney ending, but for now, he is the person who makes me laugh hysterically, takes care of me when I'm sick and makes me really happy and quite frankly, that's enough for me. This relationship has taught me how complex marriage is and has quite frankly scared me about it if I'm honest, but also that there are two sides to every story. You're right that I only have her side from emails, her Facebook posts and snippets of conversation and perhaps I judge her harshly but whilst I think it's fine that she has a boyfriend, I won't ever agree about flaunting it in their children's faces, I just don't see why it's necessary.
I find it really interesting that everyone automatically sides with the 'wife' and I do understand why. I'm genuinely sorry for everyone who has posted on here who has had awful cheating experiences, but some wives are horrid too and break up homes by having affairs first etc (like this one did), I just don't think it's related to gender regarding who's the baddie! In this day and age, I find it astounding that so many people have messaged to tell me I'm an awful person, mug etc and that he is a liar, cheat, manipulator, but very few have actually tackled the question I first posed, which was whether I should tell him how vile I think she is. It just seems like a lot of ladies with grievances want to shout at me (which is obviously fine!) but I wasn't here to have my relationship validated, rather ask for advice on how to speak to him.
As I said, I appreciate your comments and they have certainly been interesting, I might even show him this thread, but as for going further than that, I don't think it's my place to turn up in Italy or make demands like so many people have suggested! As many of you have said, teenagers are old enough to deal with the trauma and complexities of divorce and I agree that I should push him about me having visibility in his life, so thank you for that - I definitely will talk to him about it.
Thanks again, and I wish all of you the best for the future,
T xx
P.s just because you're dating some older, it doesn't mean you have daddy-issues. To the lady who posted that - my dad is great and has been there for me my whole life, including during this.