You don't get to choose. The CRA requires that the student claim the tuition amount they need in the tax year to reduce their tax payable to zero. Only what is left over after tax is reduced to zero can be carried forward to a future year.
In the next year, the same goes... if you had unused amounts from a prior year that carried forward, you must claim as much as is needed to reduce your taxable to zero... and so on.
I understand that. However, it looks like my refund was lowered when I entered my tuition, meaning that I wouldn't get any of it back, and that I would barely get anything back at all, while I also didn't earn that much.
Did you make a small amount of income in tax year 2017, or perhaps none? TurboTax was likely giving you the benefit of the Working Income Tax Benefit (WITB).
But, once you entered your tuition, TurboTax disqualified you from the WITB, as full-time students are ineligible.
More on the WITB: https://www.canada.ca/en/revenue-agency/services/child-family-benefits/working-income-tax-benefit-witb/refundable-tax-credit-working-income-tax-benefit-application-eligibility.html