Biggest scare of my life!

I am so hesitant to write this, I know someone will think that I am the worst mother on the planet, and trust me sometimes I feel like it too, but I decided that I would write it because maybe it will cause someone else to be more cautious than I was!
So yesterday we were loading up the car, the whole family was outside, since it was a sunny 52 degrees outside, despite being mid Jan. I was loading Sami in the car when Joe said "where is Josie?" She was just in the backyard, but she loves to make a run for it whenever she see's an opportunity. I put Sami down in her car seat in the driveway as I ran to see if she had gone to the neighbors swing set, which she is known to do. Nope not there, Joe was checking the other neighbor, nope not there either. Well Joe ran back into the yard to check the corners of the yard, and she was in fact in the yard, phew what a relief. Now, where are the key's, oh yeah in the house, I run inside to get the keys I come back out to see that Joe has put Josie in her car seat, oh good, now we can get on our way. Joe hops in and we start backing out of the driveway: 'bump' What was that? I feel it again. Then the most terrifying thing that I could ever imagine was happening and my mind was racing a thousand miles per hour, and I realized Joe hadn't loaded up Sami, and that I had left her in the driveway. (shaking even now as I write this) I put the car in park, scared to drive forward, and jumped to the back of the car ready to see the most horrifying scene I had ever seen. I looked down and there was Sami sitting safely in her car seat smiling up at me unharmed. I burst into a bawling mess, crying on Joe's shoulders terrified of what could have just happened, how could this happen? My poor baby, I couldn't believe what I had just done! I know that there were many angels watching out for my precious girl in that moment, I know it because I was the one putting pressure on the gas peddle, and I don't know how nothing happened, thinking about it makes me sick to my stomach! Joe looked at me and asked "are you okay?" "No, I am not!" I can't stop thinking about it and I can't stop thanking my Heavenly Father for watching out for my baby! Never ever ever ever put your child behind the car, even if you are justly chasing after another one. The thing is when I put her down I had a picture in my head of placing her on the top of the car instead, but ignored it, how foolish! I hope that you will learn from my mistake here! Please do!