Having been in a similar situation, I agree with SittingBull - try not to let your DD be too aware of your feelings - your tearful, sleepless nights, your feelings of anger and betrayal. For whatever reasons, she chose not to tell anyone at the time that her husband was a bastard - for me it was a combination of embarrassment and shock that I was in that situation, it was not wanting anyone to know about it - and also the knowledge that if I told people how bad things were, the marriage would never be seen to be okay. All the time I kept the bad things hidden, there was a chance I could make it good again. Stupid, I know, but I was too proud to admit what a huge mistake I'd made.
Anyway, your DD coped with all the bad times on her own - for her, being married was the tough bit! However hard being a single parent may be, the joy of not being with a partner who is abusive, or a cheat, or whatever your DD's partner was, helps you to overcome most hurdles.
So instead of racking your brains worrying how you could have missed the warning signs, or why your DD didn't open up to you at the time, or feeling anger towards her ex, try to quietly celebrate the fact that, as you say, she is a strong and independent woman. She has been through the worst bit, but is back on the right track. Just be there for her if she needs you, you sound lovely by the way.