You're no different than any other professional who cares about their profession and attempts to do their job the right way every single time. Take a deep breath, remember what your mentor(s) and instructors taught you, be honest/ethical, provide good support... and conclude to a value. If someone wants to argue with you, let them (within reason) and, if it turns out that you were wrong, fix it right away.
None of us are perfect. Actually, USPAP states as such, believe it or not. The comment to SR 1-1(c) - lines 450-452:
Comment: Perfection is impossible to attain, and competence does not require perfection. However, an appraiser must not render appraisal services in a careless or negligent manner. This Standard Rule require an appraiser to use due diligence and due care.
Remember that in all of your appraisals from now until the end of your career.
Good luck (and congrats) on your first assignment!