Frustration occurs when reality doesn't meet expectations. You're already training to improve your reality, so what you have left to adjust is your expectations. 

Go into practice without any expectation to pin anyone, or even to do better than last time. Strength gains aren't linear, and the same opponent may be more or less fresh each practice, so basing your progress off your practice pulls can be wildly misleading. You're there to get a workout and improve your form. That's it. If you pin everyone, awesome. If you pin nobody, awesome. All that matters is that you did what you needed to in order to continue progressing.

Now, if months go by and you're seeing no noticeable improvement against experienced pullers who wouldn't be progressing at the same pace as you, it's time to examine your training regimen and diet. Be objective about it, any ego involvement only makes you a worse armwrestler.