I understand that expenses have to be more than income however when I can only claim the interest paid on the principle and not the actual mortgage payment that cuts the expenses in half. What am I missing?
You are not missing anything. If your actual expenses do not exceed your income, you are required by CRA to report this as income. As CRA rules state that you can only claim the interest paid on the mortgage, claiming the entire mortgage payment and creating a rental loss would likely create a review of your rental claim and possibly a reassessment disallowing the loss.
Are you sure you are claiming all the eligible expenses and Capital Cost Allowance (CCA) you can in regards to your rental property? Here's a link to a complete list from CRA: https://bit.ly/2U7tO1D
There are rules around "renting below fair market value" which may apply to you if you are renting to a relative or someone you know. They can be found here: https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/rental-income/renting-below-f...