Never wanted things like this

I've been with my SO off and on for 10 years but together steadily for the past five years. He's been cheating on me the entire time! I was once so in love with him that I lost sight of who I was. Each and every time he cheated and got caught, it took a bit more of my heart. Now I'm to the point where I know I love him but could careless if he stays or leaves because my heart doesn't feel anything anymore. I got pregnant in 2012 and had a beautiful baby but our baby was born with heart conditions. Which lead to open heart surgery this past February. We separated for a short period of time after that and he was still sleeping around. I filed for child support and we were forced to take a paternity test. I had nothing to hide because I've been with this man and only him since 2011. Well with that being said, the results came back and he's not the father. I'm sad, hurt, and confused because like I said I've not given myself to anyone but him. I told him as well as showed him the results. He wasn't upset or mad. He said he don't care what that test says, that's his biological child! I'm speechless and don't know how approach this situation. What should I do, if anything at all? What is his angle? Why does he want to claim her if they says she isn't his? Could the fact that she was on a heart lung machine pumping four pints of someone else's blood in and out of her body change her DNA? I don't get it and I'm totally depressed