K.S.
You are so sweet to be supportive of her. You are also right- she needs to do this on her own. You did not let her down, she just may not have been ready before, and again- it's up to her.
The best thing you could do is go to a few al-anon meetings to gain some perspective and learn how to not be codependent or enabling (which fyi, feeling like you let her down is a classic symptom of!). You have the best of intentions but need to remember you can't do anything for her, you can only love her, which it sounds like you do fabulously.
Wish my mother-in-law waited to be asked for help!! :-)