Deductions & credits

Thank you! I feel like, since I'm on the deed and we are tenants in common and I pay half the mortgage, it wouldn't be an issue for me to claim half of the deduction. I guess my question is do I have a RIGHT to that deduction if it's a situation where my ex-girlfriend argues that she should get all of it since the mortgage is in her name. Can you help me answer that question? Thank you!