Try not to be so hard on yourself, you tried really hard and it just isn't working out. I admire you for realizing this and I am sure Coco will be much happier in a home where she is the only cat.

It takes a strong person to rescue cats, it takes a stronger person to realize that you cannot keep each and every rescue. Sometimes you have to do what is best for the cat(s) and not for ourselves. Don't feel bad, you gave it your best shot, and it is not you that failed. Coco needs to be Queen of her very own home.