A responsible partner is one that understands that she has the power to be focused, happy and strong! The mental game of tennis is very important indeed - yet often players don't take the time to exercise the mind!
The first step is awareness - what happens to you in big points? Does you mind wonder? Do you feel scared?
The second step is - what tools do you have in your arsenal to deal with your shortcomings in those moments?
If you haven't learned to train your brain - then it will be the master of you!
Don't forget when taking your lessons to also ask your coach to give you specific things to do when you are nervous! Some of mine are: - looking for the fuzz on my strings
- tapping my feet to ground myself - smelling my racket (I have rose oil on there for these moments)