You see, I think this is the problem when someone ending a relationship isn't entirely honest. The best spin I could put on this is that he has met someone new, doesn't want to two-time either of you and wants to do the right thing. But if that's the case, it would have been so much better to tell you that Blue because it would stop all this agonising about what you should and shouldn't have done. You're no doubt wise and pragmatic enough to know that if a new relationship is beckoning, nothing will stop it in its tracks and it needs to run its course. Hence, the sensible thing is to step away and rebuild your life.
You probably will never learn the truth of this, but I'd honestly be inclined to assume that at best this workfriend was beckoning and at worst a relationship had already started - and please stop thinking that if you had paid more attention or been more loving, you could have stopped this happening. Once this new relationship seemed like a possibility, the die was well and truly cast and nothing you could have done would have stopped the momentum.
There's certainly nothing you can do now, because I genuinely think this other relationship has got to run its course. It will be all shiny and new for a while, just like it was with you. Maybe it will stand the test of time, who knows? It won't be your concern.
It's possible that some of his doubts were about settling down with a ready-made family, but it would have been kinder and more honest if he had shared those thoughts with you, instead of delivering a bolt from the blue. And like I said in my first post, the thing that differentiates a relationship ending because of gradual loss of feelings for someone/genuine doubts and it ending for someone else, is suddenness.
This board has been full of threads over the years from people who hadn't a clue that there was any discord in their relationship, until they heard the immortal line "I love you, but I'm not in love with you" and IME, that generally means the person speaking the words thinks he's in love with someone else. Nothing else explains the shock, or the fact that lots of OPs didn't see it coming. It would be insane to think that so many posters had been living with their head in the sand for sometimes years and it is in fact a very cruel act to rewrite their relationship's history.
The real reason for this is rarely longstanding relationship dissatisfaction, but rather that someone else has loomed into view. When this happens, it is much healthier to reason that this is not your fault and you couldn't have prevented it. It is also why some honesty from a departing partner is so much more helpful - it really does stop all the analysing in its tracks, because if someone is seduced by a new beginning, there is nothing his partner can do. You were not responsible for his loss of feelings and it's impossible to compete with someone new.
Once you start to rationalise that this was out of your control, you can stop beating yourself up and making bargains that will torture you. You have done some really empowering and helpful things over the past couple of days and you will come through this. It is much better to reframe this as he just wasn't good enough for you or your DD. Someone else will be, if you so wish it.