Happy endings? I would say they're certainly possible, but there's no way to guarantee one, no matter how careful you are.
It sounds like he was really feeling "stuck" at the time you left him, unable to maintain "false-normal" any longer, yet unable to seek any further help. He probably tried patching up the suit of armor again afterward, and going back to pretend-land.
Certainly, you are of course very saddened at the way things turned out. I'm getting the impression that feelings of resentment and frustration are still there for you, and you couldn't keep trying to figure out where the line between what he could and couldn't help was, at the time of break-up. You two had a special bond, that no other woman in his life shared, and that's hard for the heart to forget, that YOU were the one he trusted to tell this to.
That doesn't change the fact that your time together became increasingly unhappy. Whether he was trying to "prove" he wasn't worthy of your love, or simply refusing to move beyond the first step of telling someone, isn't important. It became unhealthy, and though you still somewhere may feel that somehow you did something wrong, or didn't truly try hard enough, you know you did everything possible, and it was beyond your control; you did what was right for you.
I'd be amazed if just seeing who the email was from didn't make your head start throbbing; in your shoes, I wouldn't know whether to laugh or cry after reading it.
As far as what his story might be? I will admit to you my life has been full of me getting myself into difficult spots, yet remaining silent until a point of no return is imminent, then panicking and thrashing around, which usually has evoked some degree of tired disgust from my friends, who have tired of reminding me to "put the brakes on" before it gets to a point of crisis. You have every right, both to want to help him, and to be resentful of someone who (however unintentionally) hurt you, then had nothing to say to you for two years, then "pops up out of the blue" saying "Help me, help me, what do i do?"
He may have apologized, but is he truly aware of all you've gone through for 6 years since you met him? I'm a random stranger, but to me it's pretty obvious you (mostly) accept the outcome, certainly love him as a person, and want what's best.
It sounds like he's set himself up for another disaster, intentional or not. I feel you need to express YOUR feelings honestly to him, without worrying about whether or not he'll react well. Being a CSA survivor doesn't give me the right to force everyone to tiptoe carefully around me 24/7.
Showing him clearly how much his abuse-related behavior has affected you over time, may cause him to consider the long-term consequences of his behavior now. He's probably been smiling and nodding along for a while now, being too scared of being abandoned to speak up, but increasingly unable to be the Prince Charming I'm sure he was when they met each other. The ability to keep "faking it" will be much weaker, since he's already been honest about his true inner life once already.
Why not, just as a thought, consider either copying and sending him your post, or pointing him to this page (he may just want to sign up, if he's tired of the vicious circles). Your being "the sole holder of the truth" isn't helping him, or you; giving strength to the feeling of "I can't, or shouldn't, say anything" hurts the situation more. Abusers teach CSA victims to minimize and deny, to lie to ourselves, and others, for them.
It doesn't have to be that way- but ultimately, he is not your responsibility, and never really was. Remember, these are just words on a screen; my advice is worth exactly what you paid for it- You must do what is right for YOU.
I hope you found this helpful somehow, and wish you and your ex the best.
PS Amusing observation: I took so long with this, that upon first posting, I see indygal had already responded since i started writing, with some similar thoughts- she's just better at saying in 50 words what takes me 5 pages