Okay, so your main concern is that something is going to go wrong, correct? Is there anything about the relationship that makes you think that it might? What cues to you have that things are or could be disintegrating?
Fears generally aren't completely rational, but that's okay. We all have them! It sounds like the closer you get to actually making this relationship fulltime the more nervous you might be getting...have you lived together before?
That's totally natural (it really is!). You've both been in unhappy marriages and are so happy to be together that it's reasonable that it feels a little dream-like. As long as you don't let that fear sabotage you in some way, just ride it out and enjoy it!
Does the fear get in the way of anything now?
Wow. It sounds like he really is a great guy! It sounds like he loves and cares for you a lot and is making a big effort to show you that. I understand your concern about the daughter, but keep in mind that it could be anything. Her reaction could be completely unrelated to you. I have a feeling that in time it will bear itself out. Regardless, your beloved is doing exactly what he should be doing, which is focusing his attention on you and trying to make you feel secure.
Sometimes life actually does work like a fairytale - there are plenty of examples out there (Paul Newman and Joanne WoodXXXXX, XXXXX McCartney and his first wife Linda, etc). You're one of the lucky ones. So, unless you feel like the fear is limiting you, I wouldn't worry overly.