20 minutes of CPR is truly heroic.
Blaming yourself or second guessing yourself is a painful road to travel. I know it is difficult, but the self-inflicted mental and emotional beatings are brutal and in the end, of no use or comfort. If this ends sadly, be glad of the life you shared and be glad of the efforts you were able to perform to keep her here for family (and you) for good-byes.
I'm very sorry for you and your wife. Prayers for your comfort are in my heart.