You are playing a doubles match which you have prepared for the best that you know how. You are well rested, you have set aside practice times, you have taken lessons and you have become more competitive. Yet you end up losing the match. What would you like to happen after the match? Would you like your partner to relive the match for the next two hours? On the other hand, would you like your partner to not really go into detail about the match and go on with the rest of the day in a positive way? I believe you know the answer.
When the match is over, let it be. Do not over analyze it. Go on with your life. Loosen up! Do not take yourself too seriously. Remember that it is a game, someone loses and someone wins. Do not take it personally. Everyone has good days and bad days. You cannot plan for it. Do not burnout your partner from playing with you because you cannot let things go.
Many things in a match you can control and many things you cannot control. Let your tennis pro help with your fundamentals and strategy. Do not "coach" your partner. They will not appreciate it. Always assume you will have a "positive day" on the court. It may or may not happen, but nothing is wrong with assuming that you will have a positive experience. No one goes out on the court to lose. Everyone wants to win. It may or may not be your day. Make the game fun for yourself and for your partner, not based on whether you win or lose.
There are many worse things than losing, like not playing, not receiving the exercise, and not keeping your friends. Most of us do not make our living playing tennis. Remember to have fun.
Good luck on the courts!