How can I get my girlfriend to stop being so competitive when we work out?

You cannot "get" anyone to do anything. So i would work simpy on your own behavior during the session. When she gets competitive, make it fun instead. Maybe she will see that you are not a willing participant and stop making it so competitive. Communication can be done in many ways; verbally or through actions.  Always remember that communication goes two ways you don't have to work out her way, there are no "rules', make your own. Shift your workout style  to a more fun dynamic and do  not feed her competitive nature. Have fun!

Working out with a romantic partner has its pros and cons. One con is the potential competitive atmosphere it creates among partners. If you don’t like this, you should explain your feelings to her and see if you can come to an agreement not to compete. Another option would be to stop working out with her, and have your own separate workouts.

Competitiveness is fueled by many things. One possible thing is that you may really be motivating her to compete. She may get a better workout by pushing herself up against your level of performance. Another possibility is if you are close in fitness levels it may simply be that she thinks the competition is good for both of you and it enhances both of your workouts. On the other hand sometimes excessive competitiveness is fueled by insecurity and you may simply need to edify her. Lift her up, if she is feeling insecure for whatever reason sometimes simply recognizing her strengths minimizing her weaknesses and lifting her up for those strengths can allow her the freedom to compete less and enjoy the time with you more. Romantic relationships have many challenges but working out together can provide significant opportunities to connect and get to know each other in a deeper and more fulfilling way. Sincerely compliment her strengths consistently. It will be good for your relationship and ultimately might make the workouts a whole lot better as well.

Communication is the key. But how is it best to communicate? I find using humor works the best. Try it next time you are working out. Try one of these on "Wow you are more of a hard ass than my boss." "Somebody is getting a spanking if they keep that up." "Do you want to head to the urinals and duke it out?" "Do you want me to get a cardboard box and put your things in it?" or "Okay competitive smurf."

Competitiveness can be a good thing, especially if you let it fuel your own performance during your workout.  You could probably use a kick in butt to get you through that last set!  Think of it as a motivator and you may find yourself looking forward to your shared time at the gym.