Automatic Transmission problems of the 2005 Cadillac STS

I purchased a 2005 Sts and had it for 1 year when the transmission went out. It had just passed the 50k warranty. I took it in and had it checked, they told me there was nothing wrong with it. I had to drive with the mechanic and show him what was wrong. I went to the main company and was told there was nothing they could do. I now have an Sts sitting in my garage, not able to drive anywhere. I found out the transmission was only 1/2 full from the factory but can not prove it. I am very, very disgusted with the Cadillac company. I could have bought a cheap car that would have out lasted this car by years. .