TurboTax says my Roth IRA contribution is too high because I contributed more than I earned, but my earned income was exactly the same as my contribution. How to fix?

For self-employed, your "compensation" is your net profit minus the deductible portion of self-employment tax.  So about 92% of your net profit. 

The reason is that for a W-2 employee, if they had $10,000 of wages, the employer has already paid an additional $765 for the employer's machining half of social security and Medicare tax.  The employee "earned" $10,765 but only $10,000 is eligible as "compensation."  That calculation applies equally to self-employed people who are the employer and employee at the same time.  Your compensation does not include the "employer half" of self-employment tax.