Im Self employed. Bought a business with a loan. I know the interest payments are deductible. Do the Principle payments reduce my taxable income?

No, the principal repayments of the loan are not tax deductible. They do not reduce your taxable income. They only reduce the balance due in the loan.

 Bummer. So as an example if I had income (after deductions) of 100,000, in 2018 but I paid 50,000 of the loan principal  I am going to be taxed on the 100,000.

Unfortunately, yes. You can only deduct the interest portion of the loan repayments.