You didn't fail them. You took them in and loved them, and really, that's all that our pets ever ask of us. Your Zoey sounds like my bunch, they're always wanting to be involved in the kitchen.
I'm so sorry for your losses. It must be so hard to lose so many in such a short time. Don't be hard on yourself. Our pets can never be around as long as we can, that's the worst part about them. Just think of all the good times you had with them.