Let it go. It's not a battle worth fighting. If you really think he's cheating on you, that would be one thing. But this fight is over one comment he made to a friend while you were on a break. If I were him, I'd be less sorry about the comment than I would be that I handed you my iPad. I really don't see any wrongdoing on his behalf at all.
If you don't like the way he treats his female friends then maybe you should find a man without female friends. Your "trust issues" are your problem, don't hand them to someone else and expect them to cater to you. If you've always had these trust issues, then you should work on fixing them. As fissatore says, if you can't fully trust, then you are either a) in a relationship with someone who is untrustworthy or b) not ready for a relationship at all.
When you say "I've always had trust issues" and "being a curious woman" you are exonerating yourself from any responsibility and making HIM the person responsible for your lack of trust.
If I were you I'd look into therapy or finding ways to overcome problems with trust. Where you say "curious woman" I say "woman who can't respect a man's privacy". Regardless of how you phrase it, it isn't cute and it isn't healthy for a relationship.
My honest guess is that, until you get a grip on your trust/curiosity problems, you would find something wrong with any man you date or marry. No human being will ever achieve your standard of perfection, it isn't possible. He treats you and your daughter well and he loves you. He loves you enough to admit he was wrong even though he wasn't. He's doing a lot of catering to you already, isn't he? If he's worth holding on to, then make an effort to overcome your problems.
I am constantly amazed at how women ALWAYS very innocently look at men's PRIVATE emails & phone calls and texts.
m6898, unless there's more information which you haven't told us, I think he's done nothing wrong.
The two of you were on a break when he sent that email.
I think you have to figure out whether you have these issues because of something inside you--like have you had trust issues w/ every guy you've ever dated, no matter how the guy acts--or if your issues are only w/ him because deep down, you feel that you can't trust him?