Most people do not like being evaluated so it is natural to stress about the checkride. In reality, the checkride is just a formality. When your instructor endorses you that person is saying you have received all training required for the course and as a professional, they attest you can safely act as PIC. However, instructors do not have the authority to issue certificates so an independent examiner verifies the instructor's assessment. You will not do anything on the checkride you haven't done many times before!
To prepare for the checkride there are a few simple things you can do to build confidence:
Take a mock checkride. Have another instructor conduct a practice checkride to vet weak areas. Brush up on those items before the real deal. Two instructors saying you are ready should help you feel more comfortable.
Over-prepare. Don't just study and prepare to the minimum requirements. Try to gain deeper understanding of knowledge areas and build skill to tighter tolerances than the checkride requires.
Remember - the examiner wants you to pass. It's no fun failing an applicant. As far as the examiner is concerned you have passed the checkride from the beginning of the exam until you give them a reason to fail you. If you make a mistake, acknowledge the error and ask to repeat the task. This demonstrates good judgement and the examiner has some latitude. Multiple errors show you may need some more practice but never give up! The checkride is not over until the airplane is secured.
Follow instructions. Ensure you have all of the planning completed per the examiner's instructions, documentation is in order including endorsements and application, examiner fee, etc. Use the Practical Test Checklist in the ACS/PTS to make sure everything is completed the day prior. Then get a good night's sleep!