You have to let it go and stop torturing yourself over the past. You can't change anything that's happened, and you can't know for sure what did or didn't happen. You're imagining the worst, but have shaky evidence to support those fears. Let. It. Go. Turn the page, for your Gran's sake and yours.
I'm sorry for the pain you're going through. Regretting past decisions is a waste of time and energy that will not help your or your Granny. Instead, learn from the experience and take action now that will see to it that she is well cared for from this point forward.