You shouldn't beat yourself up waiting to have her spayed, you were only doing what you thought was best for her. What matters now is that you are still looking out for her best interests and that's what makes a for a good mommy.
If you are not wanted to use a cone, try a preemie onsize. You can turn it upside down so the snaps are on top and her tail can come thru. The one problem I had with Maggie was I didn't get to her in time and she peed right thru it! lol
I offer soft cones for surgical aftercare. They can be used for many other applications such as grooming and cutting nails. My cones are made of fabric so it's not so horrible for them. Best of luck with Poppy's spay. Will be sending good thoughts and prayers.