Does stress have a really big impact on performance?

Stress does impact performance, not just in tennis. It is normal, but just try to calm down and take a few breaths. That's probably the reason for your bad performance. Do talk your coach about tips. Also, ask your partner to give you some motivation! After a bad point, he/she could give a smile and say " good try" or something. Good Luck and i hope i helped a bit!